4 year old trained but won't use the toilet

[Chickiwick]

I'm really hoping someone can give me some advice or tips. I'm out of ideas and feeling frustrated...

My DS is just 4 and has been potty trained since October. He was very resistant when we tried around his third birthday so I waited until he was nearer 4 and when I saw some signs I took away nappies and he got the hang of using the potty in about 3/4 days.

He is dry at night - only 1 accident at nighttime ever - and has had no accidents during the day except at nursery. And herein lies the problem.

He absolutely refuses to use a toilet - even the little nursery ones. He'd sooner wee/poo himself. I've tried forcing and encouraging and bribing and he just won't go. Cries, gets very distressed etc. I don't want him starting school and having problems.

This is the last step - it's driving me insane!! Please help.