Thanks for the replies.

I've tired bribes, reward charts, putting nappies on him (which he absolutely hates). I've even brought pink girl undies and threatened him that he'll have to wear them if he doesn't start using the toilet. I've talked to him about using toilet, asked him "where he needs to do poos" and he replies "on the toilet". So he knows where he's supposed to go but he's just not doing it. (Grr) Nothing seems to be working and I'm starting to get really frustrated with having to clean him up every day.
Looks like I just need to persevere with it and hopefully he'll get it soon.