Does he do this at the start of a feed or at the end? My dd sometimes does the same. At the beginning of the feed, I think she is trying to control the flow during let down. At the end, I think she's playing around. At the beginning of a feed, she often bobs off so I'll re latch her. If its at the end of a feed, I will take her off and burp her and then see if she will latch again.