My son is 20 months old and gets severe constip...

My son is 20 months old and gets severe constipation.I have tried giving him prune juice,pears and he does have lots of fruit and veges in his diet, I have even cut back on dairy as the clinic nurse said that this can make it worse. He doesn't drink juice but loves his water. He tries going and just can't seem to get it out,he sits on the floor and tries about 10 times.He sometimes gets a little tiny bit out but thats it. It causes him stress as he is in pain when he finally does go as its quite a lot then.Just don't know what to do.Can anyone help with some advice. Oh also I did go and buy some parra-choc from the chemist as the doctor suggested it but I don't want to keep giving him this over long periods.