How often is DSS staying at your place? Is it possible that his Mum would agree to a longer stay with you guys so that you can help him with his bed wetting?
If you believe that the lack of consistancy & his mothers unwillingness to help (to tired etc, what a lame excuss!) then maybe if he was able to spend a month at your place you could try & get him into a good routine.
I am sure this is something you have also thought of & are doing but make sure he isn't having anything to Drink after say 6pm. Also I have been told that fizzy/sugar drinks are not good for bed wetters.
My younger brother use to wet the bed till he was in teens & even use to poo his pants till he was about 8 or 9 (that I know of, may have been longer but Mum didn't tell me about it).

I also have a friend who also wet the bed well into her teens.

Also with out it being meant as a punishment but maybe you coud make it his job to strip the bed & take it to the laundry to be washed. Let him be a part of the clean up so he learns that if he can't get up to go then he needs to clean it up. But I wouldn't make it as a punishment, just ask him to come help you & then he can help you make the bed again.

Good Luck with it.