Personally, Ash doesn't need any fire types. I'm sure most people who played through DP their first time didn't have a fire type on their team, but they made due anyways. I'm certain that at some point, Ash will acquire another Poke that will fill in a much needed position on his team. We just don't know yet.
If it is at all possible, I don't see why Aipom couldn't learn Fire Punch. I mean, it knows Focus Punch, so why not Fire Punch?... Well, after having done some research, he can't learn it. Period. So that eliminates that possibility. One of the few options that exists that doesn't conflict much with anything, is the possibility of Ash getting a Stunky, that evolves into Skuntank and learns Flamethrower. That's about it.
At least, to keep myself from treading on thin ice anyways.
(and in case you're wondering why I bolded "few", it's because I'm aware that there are other ideas, but there aren't many.)
~C_L~