Shutting down as you call it, is a very common behavior in kids and even adults with adhd. It sometimes helps if you present information in a different way and in small bits.
I have found affirmation of positive behavior to be a good way to present information. I don’t know how old your child is, but this works for all age categories. Like if you see your child do something positive in a social interaction, try to congratulate him for it at an appropriate time. That is a way to sort of present what you want to see from him in a way that makes him feel good and doesn’t upset him.
When trying to confront him, try to bring things up in small bits if possible and to keep the conversation simple and calm. If you bring up a whole bunch of things at once, he can be overwhelmed and that can make him shut down.