Have you discussed this with your son's doctor? If not, take him to the doctor so the doctor can evaluate whether or not there is a medical condition causing him to wet his bed.
He may be like my eldest son was. He didn't stop wetting his bed until he was 9. The doctor told us he slept so deeply he'd only partially wake up, realize he had to go, then fall back into that deep sleep and wet his bed.
I don't say this next part to alarm you, but many years later I found out the real problem. He was being sexually abused by my teenage niece from ages 6-9. He didn't tell me about it till he was 16. By age 9 he had had enough and was strong enough to tell her he didn't want to do it anymore. After that his bed-wetting stopped.
Other causes of bed-wetting are "kidney infection, or other urological problems, not producing enough ADH, a hormone that inhibits urine production." The article link below is from webmd.com. It offers a variety of treatment options.
I can empathize with you. I got so tired of soggy, smelly sheets. I wish you luck.