My wee one is the same, he can roll back to tummy but refuses to roll tummy to back, he is 4m. I just check him overnight, he has started sleeping on his side and once went to his stomach but although he can lift his upper body when lying on his tummy he won;t roll back, so I just get up and check him x
He won't sleep in anything else.
See if he will sleep in a bouncer or swing that wayhe cant roll over
From what I was told with my first, when they are old enough and able to roll over then it is safe for them to sleep on their tummy. My daughter would cry until she got herself rolled over on her tummy and then shed sleep all night if I left her to just sleep. I asked her dr about it and she said as long as there's no blankets, pillows, toys or any other loose items in bed with them then there's no problem with them sleeping however they're comfortable. If you're concerned, call the pediatrician and ask them. They'll have ideas to either help keep baby on his back or make you feel more comfortable with baby sleeping on tummy.