If it was me (which it was, a few years ago), I would go to Hamleys now, straight away.
Explain to him that you are going to choose a present now, because you know he is going to be able to do it, and that the present will wait in the bathroom until he manages a poo on the toilet.
I bought one big present and a few small presents and I put them in the bathroom right in front of the toilet. The big present was won for the very first poo on the toilet, and it was a huge incentive, then the little ones were for subsequent successes, but to be honest, once the first time was over we were pretty much away, and I tailed off to just a sticker on the jumper and then nothing at all in the space of about 2 weeks.
I definitely think having the actual toy waiting in the bathroom helped loads - big incentive, instant gratification, plus going and buying it first showed I believed we would do it, and that boosted confidence a bit. We actually cracked it the same day we bought the toy.
Good luck, I can remember how stressful this all is.