6 week old not getting full

My baby boy is formula fed and just seems so hungry all the time. I feed him 4 ounces every 3 hours or so, but he downs it so fast and wants more. So if he fusses for too long I make him 2 more ounces. Sometimes he still acts hungry after that. I try to make him take short breaks while eating and burp several times so that it slows him down. But he still seems to wanna eat all the time, then he spits up. What do I do? Do I just keep feeding him when he wants to eat even though he's overeating? Or do I limit him to the amount he "should" eat? Or is there a specific formula that might help him feel more full?