when do you give a reward?

[ChazDingle]

DS was 3 last month, have tried PT before but no success, he holds it in (for 5 or 6 hours) so he can physically do it but just doesn't want to. Have ordered a reward chart of his fave programme off ebay and am going to stock up on white chocolate (his fave) so that we can really go for it this time otherwise think he will still be in nappies when he's 15. At first i will want him to sit on the potty/ toilet at regular intervals (something he resists). Do i give him a reward for just sitting on the potty even if he doesn't do anything or is it best to only give reward if they do a wee or poo