Clare, all of mine have been tummy sleepers. Babies are just naturally more comfortable that way. I know that it's not the safest position for them but I have a sensor pad which offers some reassurance. With Mae, she just CANNOT sleep on her back due to the reflux it causes her actual pain.
Having him on his side would be a good middle ground but it didn't make a difference with Sophie. She started sttn the day we put her on her tummy for the first time! Her naps have been excellent since then.
Also, it's worth bearing in mind that having a dummy reduces the risk of cotdeath.
Also, I don't know if you remember me telling about how Mae stopped breathing? I'd actually put her down on her side that day - not her tummy!
I'm rambling now!
If you can avoid putting him to sleep on his tummy, that'd be the best bet as you'll worry yourself silly if you're anything like me
DS sttn from birth but I slept really badly as I was constantly poking him to make sure he was still alive lol!