Peeing in the potty, but not pooping in it, is a common problem during potty training. Your son is still really young, so I think I would give some more time to see if he might decide to start pooping on the potty on his own. My daughter, who is now 11, used to wait until I put a diaper on her at night to poop, but at some point, around the age of 3, she just decided to do it on her own.
I would make sure he has opportunities to see others have BM's on the toilet, so he knows it's not a scary thing for others. Sometimes videos can be helpful. Maybe being around when a child a little bit older than him goes, would be helpful too. He wouldn't have to actually be in the bathroom with the child, but after the child comes out of the bathroom, everyone could praise that child for having a BM on the toilet. ("Oh, you're such a big boy/girl now that you can poop on the potty.")
Another approach is bribery, tell him that when he does go on the potty that you will buy him something special, or do something special. Just so that he will go on the potty and realize that it is not scary. Once he goes one time, I don't think you will have trouble getting him to go again, unless he has a bad experience like trying to pass a hard stool.
If after you have given him a few more months, and he still isn't inclined to go on the toilet, then I would start things like making him at least be in the bathroom when he poops in his diaper. Also, have him sit on the potty at other times during the day, with his clothes on, to get used to sitting there, and gradually move up to sitting with pants off. Then, make him sit on the potty with diaper on to have a BM. And then hopefully, he will be able to make the transition to actually doing it. During this process, you will want to give him lots of praise and encouragement, and some little rewards, like a sticker chart, or small toy.
You never want to be negative or punishing about not having BM's in the potty. The problem is that some children, begin holding their stools, and then develop problems with very hard stools, which can even lead to anal fissures or a condition called encopresis. If you run into any of these kinds of problems, it is definitely time to enlist the help of your pediatrician.
But, like I said to begin with, if it were me, I would give him a little more time. It does not sound like this will intefere with preschool or anything like that, if you are considering it, since he is having his BM's at night. I would just keep gently encouraging him to try the potty, and make sure he is aware that everyone else does their BM's in the potty.
Here are a couple of relevant links:
http://familyfun.go.com/parenting/ch...7fatoilet.html
http://familyfun.go.com/parenting/ch...farefusal.html