LOL...he likely isn't uncomfortable because if he was then he would correct himself or wake up crying. I would just leave him. If he wakes with soreness in his neck or face then this is how he will learn. Moving him will result in his reliance on you to comfort himself. I had a little guy sleep sat up with his head hanging but he got over it after a few weeks. I just lay him down when I put him for nap and eventually he stopped sitting up once he learned that I wasn't going to come in when he screamed.