Baby falls forward when sleeping sorta folded

What do I do if my son is sleep training and falls asleep sitting up and then folds down to like his toes I always wait five min before going to put him on his back and now it’s been 1 hr and a half and he back in the San position I really don’t know what to do I want him to have a good nap but then again I don’t want him to be hurt either I did stay up there for a couple min and he is breathing his nose is free and everything

Pic for attention in a couple min I will go upstairs and check on him again