I'm another who has always been told that you can't overfeed a breastfed baby

I guess if he's drinking more than his little tummy can comfortably hold and vomiting it up, then it may just take some time before he learns to know when his tummy is full enough to stop feeding. If he was losing to much from the vomit I don't think he would happily go down for a nap, so he must be keeping enough down to stay content