If he's not fussing, I'd just let him be and see if he falls back asleep. Of course if he starts crying you'll have to try to get him back to sleep or get him up. If he doesn't fall back asleep, I'd keep him up until as close to the next normal nap time as possible, without letting him get too overtired - so the next nap may happen sooner than usual.
What is his current routine? Do you darken the room? Use white noise?