HI Dirksema, when I read your heading I had an answer ready, but I see your son is doing it anywhere anyway. That is a real problem.
When my daughter was almost 2½ I let her go about without a nappy as much as possible, so when she wanted to go, I would take her to the toilet. She did have a few mishaps mainly in her bed and on her carpet, but that soon passed. She was off the day nappy at 2½ but the night nappy was a problem and she only got off that at 3y 1mnth. Every time she did go in the loo I made a big fuss over it and I always told her the loo likes her wee and poo very much - he's always hungry and she has to feed it.
I honestly don't really have a solution for your problem but just remember that every child is different. Yours probably just don't get the message yet and when he does he doesn't really understand it yet. So when he really has to go, he just does it wherever he is. Just don't make a fuss over the mistakes, keep on encouraging him to use the loo and take him there asap, even after a mishap. What I also read is to use food colouring in the toilet water so when he wees the colour changes. That normally is a big attraction to them.
The no2 can be a daunting problem, but if he does it in his undies again, leave it and let him walk around like that for a while. He will soon find out that it's very uncomfortable and then when he wants to get cleaned, take him to the toilet and let him help you to throw it in the bowl. Then make a huge fuss over it, telling the toilet (not your son) that he's food is here, and you're so sorry he had to wait so long. Talk to the toilet as if it's a friend, perhaps that will also encourage your son to go "talk" to the toilet and "feed" him. It worked with my girl.
Just keep on trying and don't give up. He WILL get over it. If you go back to nappies now, you're in for more problems.
Good luck!
Carina

hi ya,
i understand your desperation, as was in the same boat last year, my little one has just turned 3 and is now off all the nappies, but it was a srtuggle, honestly i found that she got a fright when it came out, and that was a problem,
but i bought a book of stickers, and everytime she would sit on the potty or the little toilet seat i would give her one,even if she made nothing, as well as letting her look through it to see which one she wanted, it just meant that she sat longer on the loo and maybe i could catch a wee, but then at the school they also put her on the potty and that really helped as she saw other little ones doing it, so perhaps if you have a friend that has a child that is sitting on the potty or loo,
then i also put a sweetie pie in the bathroom as she would eventually make a pee but not a poo, but that sweetie pie worked a charm,i told her she could eat it if there was a poo in the toilet and 2 days later there was, and that was the end of it all, the night time was good, i left that only till a week ago, so she was already used to going to the toilet during the day, so i just didnt put a nappy on, and told her to go to the toilet in the morning when she woke up, and she did, amazingly they can hold it that long, and she doesnt drink anything before bed, so far so good, no accidents,
i hope this helps a bit,
i know it really gets frustrating
good luck!!!!!