As long as they get a little consolation and reassurance from one of the coaches they will be fine. Going into the 7th grade. It's almost like he needs to learn how to run. The second factor is leg strength. This athlete is carrying quite a bit of weight for his height and will need to develop the leg strength to move his weight quickly.
Strength is increased by resistance activities.
One of the simplest activities for increasing leg strength is running up hill. In youth sports endurance is typically less of a critical factor.