Hi S.,
I have a 7 year old boy and I had the same problem with him. I have a very strick schedule still with him he stops drinking 45 mins before his bed time. He goes to the bath room before he goes to bed. He goes to bed by 8:30 and I do wake him up before I go to bed around 10:30 and make him go to the bathroom. It was the only thing that worked for us, like your son mine was potty trained by 3 and during the day I had no problems but at night he just does not wake up. We talked to doctors and they all said eventually he would just grow out of it. He did not by kindergarten and was extremely embarrassed (thanks to his older sister :))so he and I came up with our own plan.
He is 7 now and going into the 2nd grade and still if he over drinks he will not wake up. He is very hard on himself about it so giving him the power to know how to avoid the accident has made him a lot more confident. Like I said all the doctors say some kids just take longer, your son is still so young that finding a time to stop him from drinking and waking him up a couple of hours to use the restrooom now will hopefully avoid him having to go through what my son did when he started school.
As far as diapers keep him out of them. I bought a plastic mattress cover to put over my sons mattress and under his sheets that way if he did have an accident we would just throw the sheets in the washer before we left for work and school. Going back to diapers is taking a step back and almost like telling him he can't do it. He can but just needs your help finding a schedule that works. Be patient it is harder on them then on us.