I hate to be blunt about this but his wieght is HIS problem.
What he eats is HIS decision.
Would you serve an obese customer the same foods?
As a chef if you worry about what your customers eat you will not be able to concentrate on cooking the food.
Until you open your own place and then you can decide the menu.
It is good that you care about him.
Maybe before he hands you the ticket you tell him that you are preparing something different just for him to try.
You can't prevent someone from doing something that is self destructive unless you are prepared to gaurd them 24/7.
Good luck with your carreer. Shalom