Don't worry too much - all babies are different.
My DS #2 was 9lbs 4oz at birth, breastfed, and gained a pound a week (yes - ONE LB PER WEEK) for the first 10 weeks of his life. Yes, he was 13lbs at his four week check up, 17lbs at his eight week check up, and so on. He slowed down a little after his third month and topped off at 33 lbs by his 1st birthday.
DS #3, nursed just as much as his big brother, got the same mommy's milk, was 8lbs 7oz at birth, and at 21 months he is only 27 lbs (a weight his big brother reached before he was 8 months old).
SO - I don't think your baby is growing like crazy...seems like the typical weight gain to me. And I don't know anything about his condition. I just wanted to give my "two cents" - I think growth charts, etc can be misleading and sometimes docs get a little too hung up on them. ;)
Good Luck!
*** Edited to add: I totally agree with the re-lactating that was suggested! That would be so much better for him, especially if he has digestive issues. And definitely don't give him any rice cereal or other solids yet, it can only cause more problems down the road. ***