Hi S. :)
Even though your son is not able to flip from his tummy to his back, is he strong enough to raise his head up? Once a baby has reached this point, the risk of SIDS decreases dramatically. To reitterate what some of the others have said, make sure there's no extra blankets, toys or other items in the crib for the baby to get trapped under. Be sure that he is not too hot in his jammies - overheating and dehydrating is a real danger for babies. If you use a blanket, use one lightweight one that is not large - just enough to cover your baby, or better yet, invest in a couple of those sleepers that babies wear, that are kind of like a really long tank top that goes all the way past their feet and zips up so they can't kick it off.
Is your baby still in your room? There are studies that indicate that sleeping with a parent in the room also dramatically decreases the risk of SIDS. In lieu of this, you can also keep a baby monitor on, and if there is something strange or suspicious, you can always get up and check.
Also, please know that there is nothing to be ashamed of in terms of co-sleeping. As long as a mom does not drink, smoke, take drugs (in particular, ones that cause drowsiness) or have a sleep disorder, cosleeping is actually safer than a crib - there are 10xs more injuries and deaths which occur from being in a crib than being in a bed. Even SIDS is reduced when co-sleeping. Most other cultures sleep with their babies - ours is really pretty weird in that we don't, and frankly, I think even if not every mom has done it every night, most moms have done it at some point, even if they are too afraid to admit it :)
-R.