M.S.
How early is he going to bed? If he goes to bed at 7 pm, that might be the reason for the early rising. If that's the case, try putting him to bed after 7:30. If he's going to bed later, like 8- 8:30, then I would do the same thing the previous poster mentioned. I have done this once, with my 21 month old, when she had been sick and her sleep schedule was completely out of whack. I went in, told her it was still night time and that her brothers, sister, Daddy, dogs, and horses were all still asleep:) 15 months, may be a little young to grasp that, but it's worth a try. You could also try rocking with him and putting him back in bed.
Good luck!