Wow, I'm impressed he can flip already! He sounds pretty strong, so I don't think suffocation would be a worry anymore. Only SIDS would be the worry. I might potentially sound insane by saying this, but if you look up "New Zealand SIDS" there is a REALLY interesting bit of information out there about the cause of SIDS that a New Zealand doctor (scientist?) and one from another country too have figured out. It's quite interesting to read about.
According to what they've found, in most cases, it's caused by the flame retardant material in the mattresses. When baby drools or gets the mattress wet, it combines with the flame retardant stuff and basically creates nerve gas. (I know, I sound crazy! But there's a lot of info to explain it and it makes sense! SIDS doesn't make sense to have no cause....).
So, what I'm saying is, make sure to change the sheets regularly and wipe down the mattress. The problem comes about when enough saliva is allowed to stay on the mattress and create the gas. So, if you keep it clean and keep the sheets clean, things should be okay. Also, having a fan in the room can help keep it ventilated.
I'll stop talking now. If you haven't read up on it (and if I sound crazy to anyone), definitely read up on it. There are also mattress covers that protect against the flame retardant material, preventing it from happening.
That's what I would do if I had a baby that kept flipping on his/her belly...