Hi,
Firstly let me confess that I know nothing about chipmunks! That's a help isn't it? I bet you feel better already.
Pacing is a classic sign of stress in a caged animal and when it is constant usually shows severe psychological trauma. ( Classic image of big cats in poor conditions in circus pens or zoos.)
As your little friend only shows this behaviour during the breeding season I would suggest talking with your vet about possible castration, of your chipmunk, not the vet. This may not be a safe option due to the risks posed to small animals under anaesthetic. He/she will also be able to advise you on a companion for him but please take into account that you will have an awful lot of baby chipmunks around if nature is allowed to follow her course.