hey, is he my ds1? Mine is now 3y 3 m. At just over 3y, he announced he wanted to wear pants. This was 4 weeks after I'd had a new baby. I thought it was an attention-seeking thing from the start, but thought, well, let's give it a go, since he's initiated it (I'd tried a few times last summer but with no success). We rewarded him with choc buttons. Well, it was awful. At first he didn't have too many wee accidents but wouldn't do any poos on the potty at all. But after 2.5 weeks or so he started having more and more wee accidents and I couldn't keep up with the washing and put him back into nappies in despair.
So, we decided to try again a few weeks later. He was virtually dry at nursery quite quickly, although he still did all his poos in his pants. But at home it was a different story - he was having loads of accidents still and I'm sure it was just attention-seeking. I'm afraid I shouted at him a few times because I got so furious. But in the last 2 weeks, since we started a star chart, we have seen a huge improvement, not just with wees but also with poos! We took him to the shops after the first week of the star chart, and let him chose a small toy (we decided we'd do this no matter how many stars he had, just to give him the idea, because we weren't sure he really understood what he was working towards).
With poos, the way I got him to start was just by taking him to the loo straight after a meal, i.e. as soon as he got down from the table, and he just went one day! Still had quite a few accidents afterwards but he gradually improved. All that worry for nothing.
TBH, it has improved gradually. At first, he didn't seem to be aware of when he wanted to go, we just had to take him. We had to leave him for longer and longer to see how long he could go, and let him have accidents so he could get used to the feeling. And we had to explain to him how to hold it in, because he didn't seem to understand that if he started to go he could avoid a huge accident by holding it in and getting to the loo quick, IYSWIM.
I hope this makes sense, I feel it's a bit of a ramble. I suspect your ds is doing it for attention partly, and you just have to find a way to give him an incentive to succeed. I have several friends whose ds's have taken ages to train too, it isn't always quick and easy even if they appear to be ready.
HTH.