No matter when/how often I call him, or what my relationship with him is, I get the "I'm at work and I don't know when I'll be done" answer when I try to call him or challenge him or invite him over or do *anything*. I can go over and talk to him at the shop, but I can't challenge him to spar or fight.
Have you tried Greeting the said Sim first? You have to do that before challenge can work.
If all fails, shift-age up the said sim until said sim dies. You will then get a new merch and challenger assigned.