Lolly, this is actually really common and not an easy one to overcome. I would pull out every trick in the book myself. First of all, make sure he is getting plenty of water to drink and foods to make it much easier to poo (sultanas and pear juice are fabulous for this). Then have a sticker chart for doing poos on the toilet, and also if necessary, you could try a pressie - either a lolly or a toy, whatever works in your family. Also give him lots and lots of praise when he does do a poo in the toilet.

He will get over this eventually. Once he has pooed in the toilet a couple of times without hurting he will be fine. I hope it happens soon for you hun. I know how frustrating it can be.