Difficulty with s blends? Help!

Hi everyone! My client is a 13 year old boy who is currently having difficulty with str blends. He consistently substitutes /sh/ for /str/ so stranger is produced schranger. How do I teach him to correctly produce str? This just started. It's really wierd because he wasn't doing this before. Thanks for any help.
He's simplifying the cluster down to two consonant sounds /sh/ and /r/ from three /s/, /t/, and /r/. Check his other /s/ blends that contain /st/ and /str/. It might be a word specific error.

If it just started he might just need placement reminder cues. Otherwise, you'll just have to teach him the placement pattern of how his tongue is supposed to move with that blend.
It's across all words that start with /str/. If it was a problem with single consonants it would be much easier to teach because there are all kinds of cues that are in my text book from tactile to phonetic that I could show him. How would I go about teaching him correct placement of the articulators for /str/? It doesn't seem as easy to break down. Thanks by the way for your response!
Think about how you produce /str/. How do your articulators move? Slow it way down as you practice.

/s/ tongue approximates to alveolar ridge
/t/ tongue touches alveolar ridge
/r/ tongue retracts

If he's 13 and neurotypical, he should be able to grasp the concept of where to put his tongue. Start with him attempting the cluster by himself with a model. Then move on to pairing it with a vowel, and then into a word.
Gotcha! I didn't know if there was a specific technique like some of the other sounds. Thanks again you helped a lot :)