My 4 month old is sleeping in his crib for the first time in a sleep sack instead of his Merlin suit. He has rolled over on his side in his sleep. He can roll all the way in both directions, but has only done it a handful of times either way. He's totally capable, just not super motivated. Should I go roll him back to flat on his back? Or let him be on his side? He does have the snuza movement monitor on.