My son has recently turned 16 and finished home ed (well to be honest we weren't given much choice). He is about to start college but is refusing to go for a support meeting. He has been increasingly in denial about his autism and another medical condition he has. He has been spending a lot of time with someone and since then he has become more and more in denial and seems to think if he pretends his conditions aren't there they'll go away and he can be normal, even though one of them, if mismanaged, can kill him. And he has been mismanaging it as his stats have gone totally astray to the point he has been put on more regular appointments at the hospital.
The problem is that he is very good at convincing professionals that everything is great because he is good at "masking". They also think his time socializing is great even though we have seen it consume him and become obsessive and co-dependent not to mention manipulative.
I don't think the law is on our side either as due to his turning 16 he knows he can do what he wants. He's already dropped out of his CAMHS appointments and is trying to get out of other things. There's no way we could prove he is incapable of making decisions even though an Ed Psych wrote in their report his mental age is lower than his chronological age all the law cares about is chronological age.
I really feel we are losing him and are powerless to do anything about it even though I'm his carer.
I'm guessing you are dealing with a family court custody situation from the words you are using. I'm really sorry I can't give you any good advice, we've got a similar problem.
It's worth finding out about whether he is considered to be a vulnerable adult, start by contacting the local authority and ask to speak to the safeguarding coordinator.