I can't speak for your son, but for me, undiagnosed asd as a kid just felt like everyone else was in on something I wasn't. Like there was this ongoing inside joke I'd never be a part of. There was a social rulebook everyone else had but you. It felt incredibly isolating, and at times downright humiliating. That is a lot for a child to carry. You know you're different but you don't know why or how to change. And as a kid, you desperately just want to fit in. Now imagine discovering your parents - the ones you trust the most to protect you - are also in on these secret rules & have been actively lying about something for years. What you think is an overreaction may feel like a true betrayal to him & might trigger further mistrust & posible paranoia: like "what else are you keeping from me?" I say this without judgment of you at all. I completely understand parents' intentions around the Santa story. I just want to offer another side for your consideration. He is not you. He will not think or feel like you. Don't expect your reactions or feelings from him, or anyone else, really. He wants to trust you. Just be honest and patient with him, and very importantly, admit that you were wrong and explain you want to earn back his trust. That even though we love you, we're not always perfect & we mess up too. This is a big step many parents skip, but it can be helpful in healing a wound, and for guiding him into being a thoughtful & responsible adult.