Regarding a scale....I've seen them online for not a too terrible price. I'm not sure how sensitive they are (i.e., if they can measure teeny little amounts or not). Regarding accuracy, IME it wouldn't matter that much in your situation. All you would want to do is check to see what the difference is before and after feeding.
Some places, like birth centers or lactation centers, will let you come in, weigh baby, feed baby, then reweigh again to measure transfer of milk.
Another way I've been thinking about a scale -- I'd like to have one on hand for my next baby, as there is potential for him to have severe problems feeding depending on the severity of his cleft -- is to perhaps get a fish scale or spring type scale. You'd have to make a fairly strong "sling" to put baby in to attach to the scale, since he's bigger than the typical newborn, but it would be reasonably priced, kwim. I've been looking at Bass Pro Shops, but any fishing store might have one. I saw instructions for this on www.gentlebirth.org.
I dunno. Just throwing that out there if you really wanted to pursue how much he's taking at each feeding.
You could try feeding so he gets mostly hindmilk, but honestly, he sounds like he's doing well and certainly not starving. He wouldn't be sleeping well if he was starving, and his diapers wouldn't be wet and soiled so much.