It sounds like you have done a beautiful job teaching him how to say the sound in isolation. You have now run into a little hangup with getting him to drop the habitual substitution he has been making for the sh sound, the /t/ sound. As therapists we run into this problem all the time. That is when we have to get creative. It always seems to be something different for each kid that helps them finally drop the sound. One thing I have done that has worked is to use alphabet fridge magnets. I am careful to distinguish that we are working on sounds not words. This seems to help kids kick the habitual substitution when they don’t think they are saying a word they say all the time. Then we put the sh together and practice making the sound, “Shhhh.” We always put our fingers to our lips to help reinforce the sound, “Shhhh.” Then I pull out all the vowels. We practice saying those in isolation (all by themselves). Then we practice saying the “Shhh” and then a vowel like “ahhh” for example. Slowly we move the magnets closer together saying the sounds each time. Finally we put them together and try to say the sounds without the substitution. I have found that overemphasizing the posture of the mouth when transitioning to the vowel really helps. For example, put your lips to your mouth when you say, “Shhh” then open your mouth really wide overemphasizing the transition from the sh sound to the wide open mouth posture of the “ahhh” sound. This seems to really help. Sometimes a slight pause in between sounds or stretching out the sounds can also help. Whatever ends up working be careful to drop the prompts, pauses, elongations or overemphasized mouth postures as quickly as possible so we don’t replace one bad habit with another. I hope this helps!