My son is four and a half with CF. He learned to pee in the potty with no problem by the time he was three, pooping was another story. It took at least six or seven months till he was completly trained. He had about 6 BMs a day like your son, and they were very bulky and greasy. we increased his enzymes and that helped a lot, but he still had trouble relaxing enough to go on the potty. We tried bribing him (yes I admit it) but that was not the answer. He finally got the hang of it once we decided not to pressure him so much. However, once in a while he actually gets constipated so bad that he'll scream on the potty, break out in a sweat and strain so hard that little blood vessels pop all over his little face!
The best advice I have to give is to be patient, buy lots of cheap underwear (if the BM is too nasty to clean up, just toss them--they are cheaper than diapers and wipes), Let him run around the house wearing nothing or just underwear, make the bathroom fun, I put his training potty downstairs in the playroom so he could as soon as he got the urge and not interrupt his play. Don't worry-he'll learn eventually!!!!!