part of the decline is real and part of it is mental.
he has to fix his belief system. he has to be more positive and really believe in his abilities.
and then I would work on the following:
1. the return
2. the backhand
3. better tennis specific fitness
finally he has to try various strategies in order to win. one of those strategies is to be willing to grind.
grind and play more high percentage tennis. that means smarter and cleaner tennis.