Not bring them home until you are sure it's a relationship?
I mean, I don't know how long you have known that guy, and how many of your friends know him, etc, but with children in the house, you have to think of more than your own safety, and you wouldn't be the first woman to accidentally get involved with a bad guy.
(Also, if you didn't introduce him to your son, then that's ... well, just bad form. When I was sharing a flat with other girls, they'd tell me when they had a boyfriend over so that I would know it wasn't a burglar when I saw a man going to the bathroom or whatever)
As for the boy catching you ... well, at age ten he probably knows what he saw, and there's no need to explain if you didn't get up to kinky stuff.
No child really wants to know their parents have sex. Let him forget it as quickly as he can.
Your problem is solved, I don't think your son will ever again enter your room without knocking beforehand.
Maybe you should have a conversation about the general fact of you bringing men into the house - your son might be upset about that, because, as I pointed out above, with flatmates you usually do tell them when you are going to invite someone.
Oh, and please don't arrange your sex life according to whether it would "put him off". A man who runs away when he doesn't get enough sex is not a man you want to saddle yourself with when you are already a single mom.