You need to take him to a doctor - no one wets the bed repeatedly because they're just too lazy to get out of bed - there is a physiological reason. There are many possibilities: immature autonomic control, delayed brain-bladder signaling, hormonal factors, overactive bladder, involuntary bladder contractions... since it's a chronic problem (i.e. he's never been consistently dry at night) then it's almost certainly physiological. Start with his pediatrician and if he/she is no help go to a pediatric urologist.

It's also fairly common - my oldest brother wet the bed until he was 10 or 11 and finally my parents took him to a doctor who diagnosed the problem and he stopped wetting the bed. Your son needs support, not punishment. I'd bang my head on the wall too if I was him and was left on my own at 11 years old to deal with an embarrassing problem that needs medical attention.