I think saying “middle line backer on varsity team” does cut it if that is the sum total of the student’s experience. AOs do understand the commitment of varsity and club sports and know that not every player will be captain or play maker.
At the same time, think about whether that is all your child actually does. Is he the team mate who brings rah rah spirit to every game, win or lose? Did he organize charity drives the team completed? Or help design/implement summer conditioning programs? If he did more than what is expected of any varsity athlete, figure out how to write that as well.
Edited to Add: What does your son think AOs might miss with the description “middle line backer on varsity team”? What part of his experience isn’t conveyed there? If he can articulate that - then add his articulation. If he loves the sport so much and wants to make sure that is clear (and he isn’t writing about it in his essay) - maybe he says, “Middle line backer on varsity team with best team & coaches a player could hope to experience” or “Middle line backer on varsity team that won the last 3 state championships” or whatever he wants to add.