If he does happen to use the potty/toilet, give him loads of praise and a couple of treats - keep up with the drinks as well.
Now, make the doll have an accident and wet her pants. Hopefully this will be before he has wet his own, if he does, follow the same as this but for him rather than the doll.
You need to make the doll wipe up the mess, and tell the dolly that not is not right, pee pees or whatever you call it, need to go in the toliet, explain to the dolly waht she needs to do again and show her a few times how to do it (the rules say 10 times, but I think that is a bit excessive). Change the dollys pants.
Have the dolly do t correctly for a reward a few more times - you could even pretent to phone Daddy or Granny to tell them that Dolly has done a pee in the toilet.
If you have kept up with his drinks, he should hopefully have either done a pee in the toilet, or wet himself by now allowing you to either praise and reward him, or have him shown again what to do.
Try not to ever get angry, you could tell him that you are dissapointed, but try to keep it that the only reason he hasn;t done it in the potty is because he hasn't understood and you have had to show him again.
Keep repeating, keep the drinks up and rewards until you are sure that he has really grasped it.
Dont allow any other distractions from showing dolly doing the toilet and him using the toilet until you are sure that he has done it a good few times - keep telling him how proud you are of him and how all his important people would be proud of him - make a point of really phoning Daddy or Granny if he actually does it.
cont/