P.M.
By 5 months, and especially after a child is S. enough to reposition his head, the danger of SIDS decreases tremendously. Particularly if there are no other risk factors.
But babies do get stuck sometimes. If that bothers your son and keeps him from sleeping, you may have to rescue him. It won't be long before he's able to get himself unstuck. And at that point, you won't be able do keep him on his back.