I don't the think ashmor as jimmy will return--but in the seasons last ep it gave you the idea that his little brother would somehow take his place (bow tie and all)
The thing is, when CK is an adult in other movies, shows, and comics; there's a Jimmy. So there HAS to be a miracle to bring him back somehow. That is if they want to keep the show as close to possible to realism. But that's me being a nerd.
If Smallville is staying true to the comics like they claim, then i would imagine he should come back. It would certainly be fitting to bring him back for a last hurray...