Wow,your son has done really well with nighttime dryness for such a young child -- which has set your expectations pretty high.
Many children who are fully trained for daytime are not fully nighttime-dry for a long time, as late as 5, 6 or even more in some cases. So your son has done very well so far, and though it's inconvenient for you that he's started wetting at night, it's not unusual, I think. Yes, the change of having company can alter kids' routines, but this may be...anything. Company, stress over preschool or daycare (any changes there?), tensions or just new routines at home, new sibling or sibling getting lots of attention, new food in his diet, whatever. Or his body may just not be as ready as it seemed.
If there are no clear changes or reasons, I'd consider pull-ups at night for a while or if you don't want to "regess" to pull-ups (some children see them as a license to forget about using the toilet at all!), put two sets of sheets on his bed with a waterproof cover in between. He wets, you strip the top set and waterproof cover, he's on a fresh set beneath for the rest of the night. Multiple layers if he wets a lot.
Talk to your pediatrician if you remain concerned, to see if there are any physical problems. But I think that progress and relapse in potty training are fairly normal and don't always have a reason or a "fix" other than maturity. This could be as simple as physical growth outpacing his physical ability to "feel the need" to use the toilet; it could have nothing to do with his training or behavior. Good luck and congrats on how well he's done so far!!