Behavior in Down Syndrome patient

I was wondering if someone can help me with something. I have a down syndrome patient who I began seeing about a month ago. His first few sessions were good, no crying, he played with toys and laughed, he even began attempts at verbalizations. All of a sudden, he would begin crying as soon as he walked in and would not stop crying for the entire session. This has been going on for about two weeks now. I've tried everything, songs on the iPad, toys with lights and sounds. I've taken him to the gym (the clinic where I work also offers PT and OT) and put him in the ball pit, on the swing. I've used a weighted vest and joint compressions, but nothing seems to work. Last session I allowed him to cry and I wouldn't pay attention to him. He would come up to me arms stretched out wide, and I would ignore him, after 45 minutes of crying, he finally stopped. He didn't do anything in the remaining 15 minutes, he just sat there and rocked himself but did nothing towards his goals. Today, mom brought a family friend's son with her (patient likes him and plays with him) to see if it would work, nothing changed, I had to ask the boy to wait outside with mom because he kept picking him up and cradling him in an attempt to get him to stop crying. I spoke to mom, she says he is not like this in school, that at home he plays independently, and she doesn't smother him. She says its only here when he begins to cry. I began taking it personally until he was seen by two other clinicians and he did the same to them. I believe its pure manipulation, that al he wants is to go outside and be with mom because the second he is allowed to go out he stops crying and is happy and he plays in the waiting room. I'm considering letting him cry himself out but I always hate doing that just because I feel its so unproductive. Mom suggested she come in at the beginning of the session to see if it would help calm him, but I know it won't really work, but I'm going to do it just so that mom isn't left with that doubt. But after that I am fresh out of ideas. I don't know what else to do aside from letting him cry and let him get used to the therapy, but I was hoping there could be other ways to target this. If anyone has any ideas whatsoever, I would really appreciate them. I'm at my rope's end.
I'm sorry you're having such a hard time. I wish that someone would create a device so you can read someone's mind in order to understand what's bothering them.

I think you should have the mom sit in for a while. Maybe he has a strong attachment to his mom? Since he doesn't cry outside the room, is there anyway you can do therapy in the waiting room? (However, he might be distracted by all the people sitting there and people coming in and out of the clinic) Good luck, keep your head up!
I would try therapy with Mom in the room and slowly phase her out. Completely in the room, standing by the door, standing in the open door way, standing in the hall with the door open and see if you can get him to adjust.

It will be good for her to see what you're doing with him anyway. But you'll need to set strong ground rules to make sure she's not interfering with your therapy.
thank you guys for your input. Unfortunately, therapy in the waiting room would not work for that same reason, he's too distracted and he won't focus. I am going to try it out with mom and see how that turns out :) thanks again!
How old is the client? Graduating mom out is a good thought. Have you tried positive reinforcement of any variety?

It sounds really rough. :/
he's 4. He doesn't do anything for me to reinforce! he literally sits there crying the entire therapy. I know he likes music, so I play videos on youtube, i play the wheels on the bus, and have him do the movements to the song, but he's crying the entire time i'm doing it, I give him a piano that makes music, etc. but nothing, except taking him out to mom, seems to work.
I agree with the above: start with Mom in the room and then as his comfort develops, phase her out. My son developed extreme separation anxiety at about 18 months. He was in the church nursery, and for a few months had been pretty sensitive, but they would page me when he would start to cry. One week someone decided he was fine, because he stopped crying when I would come, and decided to give me a break and let him just cry. When I went to pick him up, he was sobbing at the window, that was one of the most painful experiences as a parent! The next time I left him in a nursery Alone, he cried until he threw up. When he was 2, we moved classrooms, and I went in with him the whole time. After a few weeks, I went to the bathroom and then came right back in. I kept that up, gradually increasing the time when I went to the bathroom. Now he could trust me again, and so I reminded him I always came back. Finally I could leave him, and he never had separation anxiety again, but he was 27 months by that time.

Separation anxiety is not manipulation. It is developmental, and it is a GOOD thing. It is also expressing a communicative need. Having mom come in helps build the trust. Withdraw gradually to help develop independence.
Obviously different therapists and different countries work in different ways, but I always found it was helpful to have parents in the therapy room anyway. One clinic I worked at had a really strong emphasis on home programs, because parents are with their children a lot more often than therapists are, so we used therapy time to demonstrate to parents what they should be doing and usually got them to work with their children so we could see how they were going. I would never have seen a child at the clinic without their parents being present, particularly at 4 years old. Even in other places I've worked, I've always encouraged parents to come along to therapy so I could model what to do, which made home programs more effective. If he wants his mother there, use that to your advantage and talk to her about how she can work with him at home. I don't see why it's a bad thing to have her present. At worst, you might have to have a conversation with her about not jumping in and answering for him/doing things for him during therapy sessions, but she doesn't sound like that kind of mother anyway.
Luckily he didn't cry much last session. He did for the first 10 minutes but soon got over it and cooperated with the activities. When I went out to get him for therapy, mom told me she wasn't going to come in because she didn't want to encourage the behavior. She said she knew him well enough to know that he was crying because he didn't want to work. I was ok with mom coming in to the therapy, I just didn't want to encourage his bad behaviors either because I knew it wasn't separation anxiety he was suffering from. Had it been that he would've been consistent, but at school he's fine, when he's left alone in a room, he's fine, it was just in therapy that he would begin to cry and even then the first 2-3 weeks he was perfectly fine. So hopefully it continues in the next sessions.