does he eat any particular food and is sick afterwards, what would be foods he eats regulaly also drinks, is he drinking Milk?
See if any of his doctors will order a barium X-ray.
I have been looking this up recently! I just seem to be getting no where with doctors! I have seen nearly 10 different ones about this and they all keep telling me they can't.find anything wrong! .. and therea nothing he could be eating he is with me 24/7 and doesn't leave my side! I can not for the life of me work put what's going on, I just know its not normal for him to be sick numerous times everyday!
Has he been checked for an intussusception? Sometimes doctors are very slow to suspect it or diagnose it properly.
See the Mayo clinic's site - intussusception in children.
If you know it has been ruled out, is there any way your son could be eating something that harms him? Peeling paint, houseplant leaves, etc.? Is he in your care at all times?