He usually sits for a wee on a potty or toilet. He can wee standing up. We have no problem with the weeing at all. We have tried to incorage him to sit on the toilet/potty and just try and go with no pressure. He jumps on the toilet for a few seconds and then say, "I have tried, can I have a nappy please?". If we put pressure on him it ends up in tears. He say it will not come out on the toilet/potty.

Leave him in his dirty nappy for a while. Not a long, long, time, but enough to keep him uncomfortable- a good 15 minutes after he lets you know works. He can poo just fine, he probably just likes the attention of getting his diaper changed. If he is 3 1/2 he is also capable of cleaning himself somewhat- show him how to do it, and then make it known you expect him to do it by himself. Supervise him. He won't want to clean his poo if it's squished all over his bottom, and will eventually figure it out that it's a lot easier to just go on the toilet. Also, explain it to him that no one at his school will want to clean his butt for him, and that being able to go on the toilet and clean himself is a pre-requisite for getting into school. That along got my kid out of diapers in about 2 days!

Mine too! You are not alone. He was nappy trained quickly, took only a week or so but pooing time is not nearly possible.
However, I undertand that my son ocassionally have constipation, especially when he was younger. (Now, he has bananas or oranges regularly.) It was the time that we trained him while he has constipation and it somewhat turtured him. If you find the way to cope with this please kindly let me know. I'm dying to get rid of nappies.
Thank you so much,
Jam