Fall 2018
Also, it appears the whip is getting a makeover on the second season too.
This appeared as a final page ad on Issue #5 of the Kick-Ass comic.
I'll launch a crazy hypothesis: Trevor will get to battle some foe (maybe even Dracula himself) and fail to hurt him, then escape to survive. This will prompt him to seek the upgrade. It makes no narrative sense for him to attain this upgrade if he doesn't know that the standard leather whip does not hurt something that he's seeking to fight back (or maybe it does and I'm just failing to see it, but I'll stand by my hypothesis for now).
Another thing could be that Alucard informs him that the leather whip is much too weak to deal with Dracula, and they work together to reforge it (as Trevor's VA hinted they would [work together a lot] on his reveal).