Here are some general ideas for you to use on client since I don’t know him:
-Lots of leg work (lunges and squats in all 3 plains of motion, hops/jumps in all 3 plains of motion), stairs and sliding mountain climbers
-Core work (planks, flutter kicks, bicycles, etc.)
-Hills runs and some sprints, bike intervals for muscular endurance
-Rotations using cables, Keizer machines, ViPRS
-Rope work for upper body endurance
Assuming he has no medical issues or other limitations, the above list of recommendations should be able to help him get ready for his skiing trips. You will have to decide on the final selection of exercises, duration and frequency since it has to be customized according to his abilities and fitness level.
I hope this helps.