SIDS occurs in roughly 1 in 500 babies(http://www.faqs.org/faqs/misc-kids/sids/), but the rate actually depends on many factors, such as where you live, your lifestyle, your feeding methods, etc.
SIDS can occur up to one year of age, but the most common age for it to occur is between 2 and 4 months. After 4 months, the risk declines. After the 6-7th month, the likelihood of SIDS occurring is very low. It is more common in male babies than female babies.
There is a lot you can do to lower the risk of it occurring.
~don't smoke. AT ALL. Not even outside.
~put your baby to sleep on his back. If he won't sleep that way, you can compromise and put him on his side, using those wedges that keep the baby from rolling over. My first daughter could not sleep on her back due to severe reflux, so we put her on her side.
~don't use pillows, blankets, stuffed animals, or any soft bedding in his crib (or wherever he sleeps)
~don't dress him too warmly
~BREASTFEED. For every 87 babies that die of SIDS, only 3 breastfed babies die of SIDS. Reference: http://www.co.utah.ut.us/dept/healthwic/BreastFeed.asp
Personally, I would not put my baby to sleep on his/her stomach, especially if if they were not past the 4 month mark. If it was needed, I would put them on their side.
As for cosleeping, there is conflicting evidence out there as to whether it lowers or raises the risk of SIDS.
Many parent's beds have blankets and pillows and soft mattress pads, etc, that can raise the risk of SIDS, so cosleeping in that sense would definitely be riskier. However, if you make your bed "baby safe", then I would think that cosleeping would not put your baby at a higher risk and may actually reduce his risk of SIDS, as long as: ~you don't smoke ~your spouse doesn't smoke ~you don't sleep too heavily (as to notice if you roll into your baby's face ~your spouse doesn't sleep too heavily ~neither you nor your spouse are overweight/obese (studies have shown that cosleeping babies with overweight/obese parents are more likely to die of SIDS or be accidentally suffocated by their parents).
As for your son sleeping on top of you, I personally think it's fine, as long as he's on his back and there's no way he can roll off and be injured. If he's on his stomach, but you're awake and watching him the entire time, then that's okay too.
I coslept with both of my daughters, but I slept on my side, and they slept on their sides with their back up against my belly/chest. I would turn them over to nurse and then turn them back when they finished.