A couple things to keep in mind:
1. Health is a gift. If you are healthy now, don't change anything. When you come back from the summer looking the same, just tell coach that you ate 'clean', and thanks for the advice. Otherwise, ignore him.
2. As you said, your coach doesn't know you and what you eat. If he is dead set on making you unhealthy, then you shouldn't be on the team. You need to learn how to talk to him as if you care, while 'shining it on' on the inside.
3. College coaches (every sport), as a whole, have ZERO job and close to zero life skills outside of coaching (if they even have coaching skills). What would your coach do for a living if he couldn't coach? I'll answer that for you - probably working at Arby's. So, you just smile and nod for the next few years if you want to be on the team, or just walk away and focus on school (what you should be doing) if he remains an a!*hole. Either way, please don't take what this joker says seriously.