Can my 3 1/2 month old be eating too frequently?

He usually eats about 5-6oz every 2-2 1/2 hours during the day. He'll sleep from 11pm-5am without waking up, however. I know there is no rule about how much/frequently babies should eat, but all of the guidelines I've looked at said he should be able to go 4-5 hours in between feedings at this point. My mom was watching him for a few hours last weekend and decided she was going to feed him rice cereal (and before you jump on me, I KNOW he's not supposed to have it yet - my mom just thinks she knows best). He obviously wasn't ready for it, because he just spit it right back out. I'm not sure how much he weighs, since we haven't been to the doctor since his 2 month check-up. Everyone says he is a big boy. He's not very chubby, just healthy and tall. He doesn't spit up a lot or anything, so I guess I just want some reurance that it's normal :) Thanks!