I say definitely find out so that you at least know whether it's worth having to have a discussion about. If you're having a boy, problem solved, right? If you're afraid he's going to cause a scene, maybe don't bring him to the appt so that you have some time to formulate how you're going to deal with it in the upcoming months.
We had the exact same scenario with our son, except that he wanted a sister. After a couple of, "You'll still love him if he's a boy, right?" conversations that did not end well, we figured we should find out so we know whether or not the point needed belaboring. Thankfully, it didn't, because it turns out we're having a girl. Maybe you'll get lucky. :)
If it turns out you're having a girl, I think you need to drop the issue of her gender for a while, unless he brings it up himself. Keep discussions about the baby gender-neutral and try to get him excited about a *baby* period. Ask him things like what he would want to do with the baby, and talk to him about being a big brother. Get him excited for that, and he'll soon find that he can do all those things whether he has a brother or a sister. Also, definitely a gift from baby is a great idea.
Good luck! And congratulations on the newest member of your family.