You need to know when he started, how many days he gets per year, and how
many he has already taken. Else, on a year to year basis, when his leave
cycle starts, how many days per year he gets, how many days were brought
forward from the previous year, and how many was taken this year.
Lets say you have start date in B2, leave allocation per annum in C2, leave
taken in D2, then for the first scenario, in E2 enter
=ROUND((NOW()-B2)*(C2/365)-D2,0)
For the 2nd scenario, you could have leave cycle start date in B2, leave
allocation per annum in C2, leave brought forward in D2, leave taken this
year in E2, and then in F2 enter =ROUND((NOW()-B3)*(C3/365)+D3-E3,1)
If you have a situation where leave can be accumulated indefinetely, then
the first scenario should work best. However, if you have a situation where
leave may only be carried over to the next year, but is forfeited if not
taken in the 2nd year, then scenario 2 will work better.