Nina is the same age as your DS and is doing something very similar. Her feeds are all over the place and I can never work out if she's had enough. She's pulling off, crying, then latching on for a minute or so, then pulling off again crying. This has been going on for about a week now and is very frustrating. She still has plenty of wet nappies and is gaining weight so I'm not overly concerned at the moment. Does your DS have plenty of wet nappies and generally gaining weight?
Unfortunately I don't have any advice to give, but I hope it's just a faze they're going through.