Could he maybe want to burp? All my kids would start getting really fussy at the breast if they needed to burp, and often just putting them upright for a second was enough to do the job.
My DD2 as a young infant used to kick me and punch me and pull off and holler at me; she'd also thrash her body around, and yank back on the nipple. After awhile, I figured out that she was doing it when the flow had slowed, and I noticed that if I tolerated it for a few minutes, another letdown of milk would occur. Which was great for her, and great for my milk supply I'm sure, but not so great for me, because it only rewarded her for whacking on me!

The thing to do when it starts, I think, if you decide it's due to a slow flow, is to switch sides. Even if you've already used the first side, switching sides will often bring on another letdown and satisfy baby. You can switch multiple times during a single session-- this is a supply-increasing technique called switch nursing, and it's useful in the early newborn period when baby is having frequent growth spurts.