what age did your baby roll back to belly?

my son is 3 months and as soon as I put him on his back on his playmat he instantly rolls over to his belly . its good because I don't have to force him to do tummy time because he used to hate it , but when he gets sick of being on his belly he won't roll over. just fuss until I pick him up. its making me nervous because in the morning when he wants to wake up he rolls onto his belly and smushes his face into the crib . I know he can lift up his head but he won't lol