If he's close to you, $50 is fine. If you never see him, $25 is fine. And write something nice in the card!
Btw $100 is an amazing gift for a cousin's son. If you have other upcoming grads, think about that first, before you give that large of a gift. You should give the biggest gifts to the kids you are closest to. For example, all my nieces and my nephew got large monetary gifts (and I picked out a special gift that was chosen for their personalities.) I admit that I really spoiled them. There is only one more niece to go now. And I only do this for high school graduation. For college, it's a gift like one of those Lolita Love My Wine hand-painted wine glasses that has graduation caps painted on it. (Yes, the kids are over 21.) And for the nephew, a matching beer pilsner. Finishing high school is the big cahoona.
But my cousin's daughter, who I don't know well and have only seen a few times, got a check for $25.
Hope this helps!
Dawn