Student or Learner
What is he going to do duringhis vacation? Or what is he going to do onhis vacation?
I`d say : What is he going to do on his vacation, or
What is he going to do during his two weeks of vacation / during his vacation / throughout his vacation?
Harry : I am not sure you can use "for" which shows "purpose" :
What is he going to do for his vacation? He is going to borrow some money from the bank and go together with his friends somewhere in the Alps.