M.P.
Cloth diapers. You will only need to buy three tops to get you through the night and washing.
I know, it sounds daunting, but it will save you money. It's no different then washing his sheets every morning, but it's contained. Here is a link to some of my favorite diaper brands for training type pants. There are some that go up to 80 lbs so you will never run out of diapers if he was like me where my bladder didn't grow as fast as the rest of me so night time wetting happened a lot. Happy Heinies are the ones I suggest :D
http://nickisdiapers.com/catalog.php?category=16
This will more than likely save you money in the long run :D If you want any more info you can PM me anytime. I love to help out.