My son is 4 1/2, and we still use Huggies Pull-Ups. They also make night time ones, but I don't notice a significant difference between them and the day time ones. The night time ones might be slightly more absorbent? Perhaps you've already tried them, though. My son doesn't really have a problem peeing through the pull-up, and actually only has accidents on occasion, but I've read other posts where people have suggested placing a pad, either a maxi pad, or the pads people with incontinence issues use, inside the pull up for more absorbency?
I also wet the bed well into my elementary school years, so I can relate on a personal level. Your son is lucky to have an understanding mom. We don't make an issue of night time accidents and I never will. I'll let my son wear pull-ups as long as he needs them. He never has accidents in the day time, so when he does at night, I know it's not behavioral whatsoever. He can't help it.
Best wishes to you and your son. I wish our parents knew what we are learning now about night time dryness and the normalcy of accidents well into childhood.