The guideline say no soft mattresses or waterbeds. That's why I purchased a firm mattress when we bought a new bed despite the salesman who freaked out when I refused to try any soft mattresses!!
However, since you already have a bed and aren't planning on shopping for a new one, I probably wouldn't worry about it too much. I find that when I co-sleep with my babies I'm much more in tune with them. When they roll over, when they start to stir, when the cat sits on their head, whatever...I INSTANTLY wake to a semi-concious state and take care of the their needs.
I'm sure you'll be aware of him if he flips onto his belly.
Good luck!!