
Are you feeding to appetite or according to the guide on the bag. I assume he is on 3 meals a day. What are his stools like (consistency and frequency. I would say he sounds a bit light for 15 weeks as a pup of my own medium breed would be 8kg by their second jabs, and they are only 20kg as adults.
If he is eating as much as he wants at each siting, maybe he needs to go back to four meals a day for a while to increase intake, or this particular food doesn't suit him. Is it the food he was reared on by the breeder. If so I would ask them how much they would be feeding at this age.