I had/have the same problem. I have a stepdaughter who is 8...the only difference is that she lives with us full-time and goes to her mom's house mostly on the weekends. She wet herself during the day and night up until about a year ago. She still wets herself at night.
First, I truely understand how frustrating it can be...the wet clothes/sheets, constantly doing laundry, the embarassment of an older child who wets herself in public. I have another daughter who is now 5 and she has not wet herself or the bed since she was 2 years old. My stepdaughter was finally checked out at about age 6 and there is nothing wrong with her. The doctors told us that she has no physical problems and she will grow out of it. Her mother goes to the bathroom frequently, including in the middle of the night. So she just takes her whenever she goes and it doesn't seem to frustrate her as much.
After having horrible arguments (between my husband and I), rewards and punishments, etc...I just came to the conclusion that this is just how she is going to be. It could be worse...she could have worse problems, medical issues, etc. Now she has gotten better during the day, but I still ask her to go to the bathroom often. We use Goodnights at night. I now have a newborn, so I will take my daughter to the bathroom when I am up at night with the baby. I have learned not to make a big deal about it and it seems to slowly be getting better.
Also, please realize that this is more common than you think. I didn't know either, since none of the children in my immediate family had this problem. I have talked to so many moms who have gone through the same thing. Even moms that I have known for years and never knew their child had a problem. It's just not something that everyone advertises about their child.
Hang in there. He'll outgrow it.