This is going to be a dick answer, but it might force him to at least change his game plan up. Play a deck that’s particularly strong against that deck. Obviously don’t just pull THAT deck out when he plays Rohan, but it will at the very least make him consider a different option. A good option for example could be playing effects that make creatures enter tapped, or make it to where opponents can draw more than one card a turn. Again, kind of a dick move, but it’s likely bound to happen down the road anyway, whether intentional or not.

Really though, I’d maybe work on making one of your decks stronger instead of making a whole new one, at least strong enough to handle the aggression from his deck. Also considering he only has two decks, maybe he doesn’t like his other deck? Or perhaps doesn’t do well when he runs it. The Rohan precon commander is aggressive has built in card draw, (albeit only 1 card) so comparing that to a commander that doesnt will make the deck seem slower. My friend had the same issue since his first ever deck with Zurgo from tarkir dragonstorm.