I really have no clue what approach to take any more! We started potty training nearly two years ago when he was asking if he could wear pants. Since then he can occasionally go for a whole day with no accidents, and can sometimes either ask to go to the toilet or just go by himself. But more often than not we have to remind him to go.
But he often just refuses to go. He can be sitting there wriggling his legs and clearly needing a wee but he is adamant he doesn't. If we take him to the toilet he will stand there holding his willy but nothing comes out, then ten minutes later he will wet himself on the sofa.
The other day we were trying to persuade him to try for a poo. He sat on the toilet for a while, and said he didn't need one. Then he refused to put his trousers back on again so we left him bare-bottomed. A few minutes later DP noticed a poo coming out, just about to land on the living room carpet!!
We have tried reward charts, loads of praise for using the toilet, ignoring it, getting him to change himself after an accident, etc and I have no clue what is the best thing to do any more.
When we pick him up from school he has normally has a poo accident and he often smells of wee. As does our sofa 
Sometimes I think he is being deliberately naughty and maybe we should tell him off/naughty corner/tell him he can't watch TV later or something. But then I wonder whether there is some other issue and we should be more gentle with him. We've been to the doctor and he's had tests which show there isn't a physical problem.
Should we be harder on him? Softer on him? Ignore it? Make more of a deal of it? Any ideas???