Hi cheekypepper - kittywits is completely right....the worst thing is to get stressed out with all this potty/toilet trg lark. Kids so easily pick up on this and I'm certain, even at their young age, they play on getting your attention, whether good or bad.
The absolute worst thing you can do is put a child in pull-ups - in my view they don't help whatsoever - they are an expensive version of a nappy - I'm yet to find a child who treats a pull-up as a pair of knickers/pants - neither of my children pulled a pull-up up or down to do a wee - they simply wee'd in them - they don't know they are any different to a nappy. The only use I can think of with regards to a pull-up is they are handy to put on a newly trained child if they're having a nap, which I still do with my DD because she has no bladder control when she's asleep!
My advice to you is pretty similar to kittywits - put your DS in some trousers that are easy to pull up/down - no jeans, nothing with a button or zip - just a pair of jogging bottoms or anything elasticated. I would try putting pants on because he'll have to get used to wearing pants but perhaps leave pants till later. Have you tried putting something in the loo for him to try and wee on whilst standing up or even sat down - try a bit of coloured paper or something that will flush without blocking the loo? Use a step for him to stand on and he'll find it great fun to wee on a target!
I must admit I used stickers with my youngest DD and it did work - she thought it was great fun to stick her stickers on the loo seat each time she did a wee. Chocolate works too and I don't see it does a child much harm to have a treat for doing a wee!
The most important factor in all of this is DON'T GET STRESSED.....very easy for me to say but it simply isn't worth it - also let him lead the way - if he's not ready to use the loo - so what? Don't worry about nursery - you've got a long way to go before he starts. I found with my DD that if you try too early it puts them back - I spent the first week of the Summer holidays pulling my hair out with her and I was even beginning to wonder what was wrong with her - why wasn't she like other kids her age...and younger who were out of nappies. Then once I decided I'd had enough and let her get on with it on her own - let her wee in her knickers, soak the floor etc etc - she soon got used to the idea that this was not a good thing to do because I took no notice of her, didn't shout at her, tell her off - I simply cleaned the floor and walked away yet when she did a wee on the loo I made such a big fuss over her and she finally clicked and there was no looking back.
So after all that waffle here's my advice...don't get stressed, let him lead the way, let him decide when HE'S ready (not you or nursery), don't make a sound when he has an accident but scream for England when he uses the loo/potty......good luck...!