R.D.
If the doc says he's fine, he's fine ;)
My first daughter was a peanut the first 4 years... she's still the shortest kid.
My second always grew up, then out, up, then out, so she roller coastered all over the board (90th percentile, 30%, 80%... you get the point).
My son was a chunky monkey forever, with the biggest head (think: Charlie Brown)... he's finally growing into that big dome piece of his, but sadly, he's losing the chunk (so the nickname 'fat man' isn't really sticking anymore).
My husband was THE FATTEST KID... the second he hit puberty, he was like an iron strong bean, all lean muscle and not an inch to pinch... still is ;)