Do not let him drink anything two hours before going to bed and always get him to go to the bathroom before going to bed. If that does not work  don't let him drink anything before going to bed,make him go to the bathroom before going to bed and after 5 hours of sleeping wake him up and tell him to go to the bathroom.

I have had a little experience with this and all I can suggest is taking him to the doctor to be checked just in case but if the doctor can not find anything wrong I am afraid you just have to ride it out. my experience was keep them in pull ups and do not put any pressure on them try what the member below suggested with getting him up a couple of times during that night I also found this helpful.
Some children just take longer to night toilet train than others. Good luck with it all

My 5 yr old still occasionally wets the bed, its going to happen......  Like your doing no drinks at least 1 hour before bed, toilet before bed, we get our son up about an 1 1/2 hours after he has gone to bed and take him to the toilet, now depending on when you or your hubby goes to bed take him again before you go, if we go to bed early, if I get up in the middle of the night I will take him to the toilet, but I rarely do that......  You could put him in night pants so if there is a accident he doesn't wet the bed, but other then all that it comes down to been patient....  My son wets the bed because he is a very deep sleeper, a bomb could go off in his room and he wouldn't stir.....  I keep getting told by people with older kids that they do grow out of it unless they have a medical problem..... x