Ahh bless, I'm sure your OH wouldn't cheat on you, despite what other blokes might do - some are just scummy and have no morals - I'm sure your OH isn't like that!
My OH hasn't had any sex since the 2nd trimester, and although I feel bad about it I just have got no interest in sex at all right now. He understands though and is being fantastic about it!
If you want to then I'm sure as long as he's gentle and you try it in different positions until you find whats comfortable for you then it will work well, but if you don't then don't feel bad, you're carrying his baby after all! x

Try to relax and not stress about it when the time comes. If you haven't seen your OH for awhile, you probably won't be able to keep your hands off of him and sex may come very naturally without the worry.