hi
i had this when i was training my daughter. She used to go for a wee in the loo, then go downstairs and poo in her potty
I think its just a matter of time really, you are doing everything right so far so i think he'll just do it when he's ready. Meggie just one day decided that she wanted to poo in the big toilet and that was that.. sorted.
good luck and try not to stress too much, i know thats easier said than done tho huh!! xx