I have been a step-parent for a few years and one thing we have to remind stepchild EVERY visit, there are rules of acceptable behaviour at mum's house, and there are rules of acceptable behaviour at dad's house. Once we point this out to her, she's fine. Swapping between the two households always throws her for a day or so of each holiday.

I realise this is probably no use to you (with kid having autism) but i just wanted to morally support you that different ways of living happen in every family setting, you are not a prude, and totally entitled to set boundaries in your own home.

I think you are acting responsibly but agree professional counsellor help is needed. For all of you, the stepchild needs training and guidance on this behaviour, and you need to feel safe with him in the house.