Quoting :*:CHRiSTiNe:*::" Deal with it. He's only 3 months old so expect many worse nights than that. Be happy its only for an hour."
This. My 9-month-old still does this some nights.
Don't turn on the light or anything stimulating as others have said. You could try rocking him back to sleep. Sometimes this works with my son, but usually it takes me just as long to rock him back to sleep as it takes him to put himself back to sleep.
Is your son crying while he is awake? Or does he just play and go back to sleep? As long as he IS NOT crying, I would just leave him in his crib to play and tire himself out.