I would definitely talk to his doctor about it if you haven't already. I wet the bed for years as a child due to a bladder condition that finally had to be corrected with surgery when I was 8. Make sure he's not getting UTIs.
Here are some things that my mom used on me that helped:
Don't let him drink more than a couple sips at a time after dinnertime (for me it was around 5pm), and nothing caffeinated past lunch-time (around noon or 1pm). Caffeinated drinks shouldn't be allowed often anyway; caffeine stays in your system for up to 24 hours and can really contribute to the bed-wetting.
Take him to the bathroom at least twice an evening (once after dinner and once before bed), and make him stand/sit there for 5 minutes trying to go, to make sure it's all out.
During the rest of the day, try to get him to go to the bathroom (even if he doesn't feel like he needs to) at least once every two or three hours. He may not be understanding or even getting the message from his brain saying that he has to pee until it's too late, and going this often lets his bladder get used to being empty... He should start recognizing when his bladder feels full, and that will help him at night.
Cranberry juice and water are always the best things to drink for kids with bladder issues, so try to stock up. ;)
If none of that improves his bed-wetting, then middle of the night trips to the bathroom, again making him try for a full 5 minutes, may be in order.
In the meantime, if you want to try any "big kid diapers" or plastic sheets or whatever will make your life easier on the clean-up end, feel free.
*makes sure to save all this so it's handy when she has to deal with it in few years* (Both my husband and I wet the bed as kids, so I'm fully expecting to have this come up with our daughter.)