My brother told me you could battle Ash in one of the main Pokemon games, but when I asked him which one, he said he doesn't remember. So can you battle Ash in one of the main Pokemon games? I don't believe my brother. But if you can, what game?
No. There's a common (but untrue) belief that Red, who is at the top of Mt. Silver in HeartGold and SoulSIlver, is Ash. This may be what he's referring to.
Ash doesn't exist in the main games. Anime =/= games.