I would definatly introduce him as a friend first if your child is younger, and if your child is older don't play games, just be honest and tell them up front. I would talk to your son about it beforehand, either say my friend/boyfriend is coming over. If he's younger take him to a park because as much as you're anticipating your son's reaction to him, you should anticipate your boyfriend's reaction to him as well. Then you can see how they play. If your son is older, you should still do something together, maybe out to dinner so you can get to know each other.
But if your son doesn't like him I would take that as a very serious thing and give the relationship a BIG thinking about.