I am not a doctor, nor do i have any medical training, but . . . . .
have you had him checked for things like intestinal problems or spasms in the bowel etc chron's, divaticilitis(sp) etc
have you taken him to a paed???
is it possible he is sick?
In kids there are small glands around the belly button. If they are fighting something, these glands flare up- the pain is as bad as appendicitis. Perhaps he is getting bouts of this and his glands are up for a reason?
maybe there are other foods that are annoying him- maybe get allergy tested?
next time he is screaming , i would rock up to the emergency room of the RCH and ask for their help. They can at least run tests to see if there is a reason
He can't go on like this and either can you!
good luck and let us know how you go
odette