This is a challenging problem without easy answers. I myself wet the bed until I was 11, with great shame, so please avoid causing any more shame for him then he already feels.
There are a number of things one can try. First, does he consume dairy? I drank lots of milk when I was young, and as a grown adult, I have discovered I cannot consume dairy without physical repercussions. (Research verifies this can cause bed wetting...was that the cause of mine? Who knows.)
Gluten is causing a lot of people lots of problems, too, and is certainly worth checking out, although not easy to do, it IS possible.
It could be any other allergy, or food sensitivity as well as environmental chemicals in the room or home.
I am now studying hypnotherapy, and have learned that this modality has proven helpful in resolving bed wetting, too. This could be as simple as guiding the child, as he/she is drifting off to sleep, with imagery. Their care givers can learn to do this for them.
I am also an herbalist, and if this were my child, I would try tonifying the kidneys with some safe herbal tonics.
Having wet sheets every day is a pain, but understand that this is a bigger problem for him then mere wet sheets. There are many avenues to consider as to the cause, and it could be a combination of causes. Dont let it become a control issue, which will only intensify it.
I hope this is helpful, or at least gives you some new thoughts to consider.