In the last couple of months I've heard a number of stories of parents who took a calculated 'don't beat them, join them' approach. These were all in cases of girls insisting they are boys, which is recognised as social contagion, so I don't know how well it would work on a son. But what has supposedly happened is that once the daughter claims to be trans, one of the parents also comes out. Either the dad says he's relieved to hear she is trans as he has been wanting to tell her that he is really a lady, and now knows she'll accept him. He then starts dressing as the worst example of an AGP and starts insisting he take her to school, out for food, shopping together, etc until he embarrasses her back to reality. Or the mother says, 'wow, your reasoning for being really a man apply to me too, I must also be a transman.' The daughter will most likely hate this and start arguing the case for her not being trans, and will eventually realise it's bullshit.

In both cases it means that the parent gets to dive right into the cult with their child, go to the meetings, can't be cut off by the school, etc. It stops the child from being isolated, while at the same time being presented day in, day out with just how ridiculous they are being.