Bedwetting in children under six years old is considered normal, I'm afraid. Most children outgrow it on their own. Nobody wants any extra laundry though!
Until your 5 year old stops, how about asking him to put his wet underwear in a place that won't bother you so much--like a plastic sack or the laundry room? Buy him an extra pack of underwear until he's done with this phase.
There are some other things you could do that might help. Make sure he is getting enough sleep. Don't give him chocolate, iced tea or soda, as these can irritate bladders of all sizes. Have him drink his water for the day before dinner, and eat dinner early as you can. Then make sure he has enough relaxed, boring time after dinner to use the bathroom before going to bed. You know kids don't want to go to the bathroom when more exciting things are happening. You probably already know that punishing kids for wetting doesn't help. They really can't help it.
If you are interested some information on treating bedwetting with homeopathy, here is a short article I wrote.
http://www.squidoo.com/homeopathy4bedwetting