Both of my babies slept on their bellies. The first one because there wasn't this huge "back to sleep" campaign back then. The second because, like yours, he wouldn't sleep any other way. We co-sleep and he naps in our bed on his belly by himself. I didn't allow him to sleep by himself on his belly until he was able to lift and turn his head but I think he was younger than 3 months. I read something in my ped's office last time I was there about how it's better for some babies to sleep on their bellies if they have a medical issue. One of those issues is GER or acid reflux. I never asked my ped about it because I didn't like the testing that is done to diagnos it but I suspect that my baby has this problem. Anyway, my point is that I've done it and my babies are now 13 years and 5 months. I know it's hard not to worry but try not to. Especially if you are sleeping with your baby, he should be fine.