You've made some incredible assumptions here that OP never mentioned at all. OP never hinted that they're putting pressure on him, or making him feel like he's doing anything wrong. You're making those silly assumptions. Additionally, the fact of the matter is that his own attempt at speaking is preventing him from speaking, that is the crux of the issue, and why speech therapy is being administered. Rightly so. OP, to provide some relevant input, it sounds like you're doing the right things already. A large component of how you can be directly involved is by providing a sense of reward to your son upon a moment of fluent speech. Like "Hey you said that really well!". I would avoid speaking for him, especially in nervous situations when you may think speaking for him is the better option. At least for a couple of years while his speech system is developing.