There are no less than a hundred excellent whiskies under $100.
Do you have any idea what he has had before and enjoyed?
Without knowing any details about his taste, I will recommend my "crowd pleaser" recommendation: Aberlour 18yr. It has the smoothness and profile of Macallan 18, but at nearly half the price ($80-90).
Or try the Dalmore Cigar Malt. I love cigars and was given a botte as a gift. I thought it was really thoughtful and a very good whisky.
I now prefer the Dalmore 15 to the Cigar Malt, but marketing is marketing!