There's really not a whole lot you can do. I still remember some of the kids in first grade having trouble
with that.
It's perfectly normal for relapses to happen. My daughter has had her share of relapses and she's 5. We usually ask her at night if she wants her panties or "night-night pants". Since she's 5, the "night-night pants" are getting a little on the small side for her and getting a little uncomfortable. We're looking at that as "motivation" or "incentive" for her to get up and go potty. She's been doing a great job, but there are still rare occasions where she's super-tired, where she might wet her bed because she's so pooped.
One night she wet her bed, and was so upset that she tried to clean her bed with wipes, then changed into pull-ups, came and told me her pajamas were wet. She was so upset that she was on the verge of tears thinking I was going to be mad at her. I just calmly asked what happened, and said, "Well it looks like you wet your bed - let's get it cleaned up." SHe said, "You're not mad?!" I told her no,I wasn't mad...I wasn't happy about it, but I wasn't mad. I told her that anytime she wants to go potty in the night she can come use our bathroom and she can do it anytime she wants. And the calmer I've been with her about it, it seems to work better. She's even been sick this week, and has been coming into the bathroom about 4am to potty. I keep telling her, "That's a good idea...I really like it when you use the potty during the night. You're doing such a good job!"
Okay - so here's a few things that might be worth considering trying:
**Limit drinks in the evening. If your son goes to bed at 8, I'd say nothing to drink (sips of water or something okay, but no big cups of drinks) after about 6:45-7:00 p.m. That's helped a lot with us - it was a suggestion my sister gave us (she has 2 kids).
**Encourage going potty about 2-3 times before he actually goes to bed...work that into the routine. "Okay we're going to read a book, but I need you to go potty first."
**Give him the option of pull-ups or big-boy pants, and don't make a big deal about it either way. If he complains that the pull-ups are uncomfortable, just let him know he can wear his big-boy pants, but he might have to get up and go potty in the night so his bed stays dry.
For what it's worth - and good luck!!