I also think it depends upon the age of your son. Since you say you both discuss your disorders and try to give each other support, I do think he's old enough to discuss this situation with you.

First, I'd put whatever agreement you two come to in writing so both of you have it to refer to later, and it won't be "but you said but he said" situation.

I would allow him to go, for a limited time (agreed upon by both of you.) By having this is writing, if his father tries to say you gave up custody, you'll have it to prove otherwise. (Unfortunately, you never know.)

I'd make sure it's you son's desire, and not a push from the other family. I doubt you'll find out by asking yes or no questions, but by allowing him to share all the reasons he wants to try this.

I would also have him agree to go to his therapy. Regardless. If his dad won't comply, then he can't go. Period. Why would you think anything less is good for him? You already know how tough it is to deal with such disorders, and you're thinking of letting him quit the professional help he needs to survive in the future on his own?

(You need to get it in writing from the dad, if he agrees. Notarized?)

You need to agree to contact between you both, and time. You both need to do things together, not a 30 second phone call once in a while.

I'm sure you can think of other things to discuss with him, about his expectations for a good outcome in this, and how it might not work and how it won't be his fault etc?

Good wishes.