did you wet the bed as a child??
sometimes its just in the genes!! seriously i never wet the bed and my daughter never wet the bed,
but my older sister did till she was about 10 and strangly enough her son does to, well he has stopped now but did till he was 9-10 as well,
she wouldnt give him anything to drink before bed but he would constantly have accidents.
sometimes its just a case of passing these things down, did the father wet the bed???
there could be a reason behind it but if you cant find one, dont worry too much, i know its a pain, i remember from when i used to look after my nephew!!
you can get special sheets for the bed, id try and encourage pull ups though, he is still young and saves him getting up all smelly in his jarmies
good luck xxx