Have you asked him why he is waking up? My son is 3Y10M and this is a prime age for bad dreams. Actually for us, it was last year. It took us MONTHS to figure out what was going on last year, somehow he finally told us is was bad dinosaur dreams. Even now, he won't necessarily tell me what the bad dream was about-maybe his way of trying to make it go away? So if this is his case, then you should be comforting him, rocking him for a bit, and then putting him back to bed. Does he have a security toy or lovey? All would help with the falling back to sleep. It usually takes two trips to his room for K to completely get back to sleep.
But...if your son is waking up at the exact same time each night and you eliminate nightmares, then it has become a habit. It is amazing how quickly they can develop one. So it needs to be broken. Can you just call to him and tell him to go back to bed and that you love him? Tell him it isn't up time/time to get up yet? Does he still take naps? If so, then maybe he doesn't need them anymore. This age needs 11-12 hours of sleep.
I hope that you are able to find out what is going on quickly so you can get back to sleeping through the night!