Gah!! My ds has been exactly the same.
Even now, at 4.4 he will often have to be led by the hand with his but cheeks clenched together to the toilet because ha just won't take himself.
It is beyond frustrating.
As much as it pains me to do it, we have found that he only thing that truely motivates him is payment :/
Chocolate is the currency of choice here.
Can you up the reward ante?
At first, I had a whole prize box going on , with cars, smarties, poppers s d cuppa chips.
That really kicked things off, now I can get him to go for as littles as 3 smarties lol.
Also, if we are going somewhere really exciting, I tell him we can't leave until he does a poo, that works too.
It's crazy hey ?