[Aceium]

Let's assume that the bottom of his range is A9s.

Holdem Hi: 990 enumerated boards containing 9s 4c 5h
cards win %win lose %lose tie %tie EV
5d 4d 687 69.39 303 30.61 0 0.00 0.694
Ac 9c 303 30.61 687 69.39 0 0.00 0.306

You're getting about the right odds to call against A9, but you're not that far ahead. If we are assuming that this is the bottom range that he'll make this bet with, then you are probably a little farther behind. In that case, this is starting to look like a fold.