As far as sleeping on the tummy goes, I say follow your gut instinct. I know a lot of people think that the back is the best position, and while that's probably true for many babies, it's not true for all babies, such as your own. I would definitely put the baby on his tummy to sleep if that's what his doctor recommended.
I opted to let my ds sleep on his tummy from the time he was a couple months old because it's the only way he would sleep for longer than a couple hours at a time. He was very strong with his neck also, so I didn't worry about him doing a face plant, and not being able to lift his head to turn it.
I think you should follow your dr's advice on this one.
Whiskers