Hi!
If his lipstick comes out and retracts back in easily I wouldn't worry about it. O' the joys of having little boys
If it doesn't go back in and starts to look dried and even discolored it's called Paraphimosis. Sometimes it's just the tangling of hair around the base of the penis that is keeping it from going back into the sheath. Other causes are more serious like infections, congenial abnormalities, foreign bodies and tumors. Either way, if his lipstick does not go back in, it needs to be seen by a vet. The issue can cause bladder/urinary trouble. Often times, cold compresses to reduce any swelling and dextrose solution applied to the penis by a vet may help it go back in.
Sounds like he is just being a little boy dog though. If his lipstick goes back in, I wouldn't worry about it.