My youngest was like this also. I used to sleep her wedged on her side. She was a very chucky baby and used to wake herself up all the time. She slept much better on her side. She slept right next to me in a side car situation so I was happy taking the risk. I started sleeping her on her back when she was 3 mths as she could wriggle quite a distance at that age and I wasnt happy with her staying on her side. Here started 6 mths of dreadfull sleeping, waking upwards of 5 times a night on a bad on for 1-2 hrs at a time!! She finally started sleeping better when she learnt to roll at 9.5 mths (yes you read right, she was crawling and cruising before she learnt to roll) Since then she sleeps on her side or tummy, I hardly ever catch her on her back.
So after all that LOL My advice is , yes it is a SIDS risk, but if you are happy taking that risk and are minimising it by being present when he is asleep then it is really up to you.
Mumma to DD1 Jan 04 and DD2 Mar 06