At his age, he should be getting around 32 oz of formula per day. Since he's both breast and bottle fed, it might be harder to figure out just how much he's getting, but it sounds like he may be a little low. You might try increasing his formula a bit, and once his intake is a bit higher, you can worry a bit less about how much solids he eats. 6 oz is a lot of solids, but some babies are hungrier than others and that's okay!