my bub does that too, and he's almost 10 months old. I read somewhere (can't remember where) a little trick that works sometimes for me when I need to put him down, when I know he is not feeding but just kind of sucking while asleep, I put my finger on his mouth at the side to release him, then press lightly on his chin. He then just closes his mouth tightly and stays asleep. I don't know why it works, but sometimes it does. Usually I just wait until he stops though