First of all clean the mattress (on a sunny day as to dry completely) use a bucket and enzyme cleaner (this can be found in PetSmart, WalMart) generally in the area of rug cleaners comes in a spray or a larger size.....Follow directions, then let dry in the sun. Double beds waterproof covers are hard & expensive. Scotchguard the heck out of the mattress after it has dried. Then, go to your local Lowes, WalMart and look for a large "tarp" that you can tuck in around the corners, or I would even try a canvas "tarp" and scotchguard it - generously, (let dry and repeat). I like the idea of the canvas because it won't "crinkle" or make him warm during the night. You can also use the pampers or others that make "big boy" pants just for those in between times. Cut off all liquid 2 hours before bed, have him sit on potty even if he says he doesn't need to and let water flow gently in sink. If he needs a drink, sips or ice chips to melt in mouth only. Scotchguard is a wonderful product and the generics work just as well, I still use them on my car seats and my husbands, and on white dresses, or special occasion dresses. My friends have loved this idea. No more little missed stains that show up a few months later....Good Luck! I'd also let him put swim goggles on, and a little paper mask (97 cents) at WalMart, or Lowes, etc. and have him "do" the application-sometimes being a part of something feels like he is in "control" instead of being treated like a "baby". (You can always re-do when he's playing at a neighbors or at school...). I have found that letting kids take control of some things helps them to "buy" into it. All my children eat various foods, they have cooked once a week since they were two. (with my help!) Pot pies made w/premade pie shells & leftovers, "Dips" for raw or blanched veggies (each one loves vegetables) they made all kinds of dips and I bought the small containers from a supply store so they sometimes had 6 each to choose from- but veggies got eaten...mayo & ketchup mixed are quite different to a childs eyes....mustard and honey (yes, you can buy it but them "making" it makes it special)....Getting back to your son-I'd call it "the plan" and sit down and write down what you'll need, help him measure his bed - plus down the sides to "tuck it in"; etc. He'll be thrilled-not embarressed....Have fun-one little lesson....