Some children just have problems holding it at night especially boys for some reason. My brothers and my uncle all wet the bed until they hit puberty. There isn't much that can be done and to make a sad story short...I had a cousin that died from a medication given for bed wetting back in the early 80's because it was experimental (good ole military doctors didn't inform us of that) and it built up in his system.
Not sure what problem you have with restricting liquids close to bed time???? Your son is not going to dehydrate or suffer physically. My mother restricted my brothers and if they complained she calmly told them that it was something they had to deal with because of their accidents and that was a fact of life. Life is not fair. His brother doesn't wet. For some reason that is not his fault, he does. FAct of life. WE all have to live with and accept inequalities in our life.
At any rate, my mom controlled most of the accidents with these steps.
1.) restrict liquids after five o'clock or so. The boys got a half a glass of milk with dinner.
2.) Set the alarm for two times during the night to get my brothers up to go to the bathroom.
3.) Certain foods were off limits at dinner time like watermelon and soup.
4.) She had the boys help her with the sheets by stripping their beds if they had an accident and putting them with their clothes in the washing machine. They also helped make the bed before bedtime.
I know there are lots of things that you can buy like alarms to help them wake up. You can try them if you have the money, but ultimately it will take time to grow out of. Just be patient and diligent helping him deal with it. All of us have things to overcome and this is a good teaching tool for him. It is not his fault, but he has to do what he can to overcome it without whining about it. It is a good tool to teach him to be a big boy and how to overcome obstacles with grace and will power.