You're not going to like my answer
DS1 started doing wees standing up at the loo at 2.7. Never bothered with the potty, just copied an older friend and baely had an accident after that. So far so good
Poos however were a different matter. Point blank refusal to go near the potty or loo. Hysterics ifI pushed it. Requested a nappy for every poo (so clearly knew when he needed to go). I tried treats, sticker charts, bribery etc etc but nohing worked.
After a while I was advised to withold the nappies to see what happened. What happened was that he would hold his poo for 3/4 days, be white as a sheet, be off his food and groan in pain lying in his bed while trying to prevent this rock hard monster poo coming out of his little bottom. Horrendous.
In the end I reverted to the nappies as I couldn't see him in pain.
One day he just walked out of the livig room and said "Just going for a poo on the loo Mummy" and that was that. I could not believe it. We have never looked back.
The down side to all this though is that when all this happened he was 3.10 so 15 months after he was initially trained.
We are probably an extreme case but honestly, it will happen when your DS is ready and if it takes forever then I really wouldn't worry, I promise you he will eventually get there!