This is very normal and I believe we had a post in a few days ago asking the same thing, it is a bit of jealousy I think even if he had been the bio Dad you would get the same response, let him see this often do not stop, it is good for children to see love ,let him join in as you say ....a threesome cuddle each time so he knows he is part of you both ...good luck he sounds like a lot of fun, make sure his 'now Daddy' plays games and has fun with him....