Unfortunately, the only thing I have to go on is what was shown on-camera. As of ToGoM, they did indeed have a lunch date. But it ended badly with Mayson accusing Clark of going to lunch only to ask her about Bill Church and for no other reason.
For all Clark knew, their relationship never got off the ground. I wouldn't even count that as a date considering it lasted no more than two minutes. Mayson may have interrupted a dance at the end, but it's fic-writers who implied that she monopolized Clark for the rest of the night. For all we know, Lois had the next dance since they did come there together. Clark isn't the type to abandon his "date."
They weren't seen together again on a "date" until Top Copy. And that wasn't a date either, since Lois was there and the goal was to pump Mayson for information about the Intergang defector.
True, Mayson asked him to the mountains in TEHI, but it seemed to take Clark completely by surprise. Could be his old-fashioned rural upbringing that surprised him, but there had been no indication beforehand that they'd even talked to each other since the dance at the end of CoM. And Mayson didn't say she was inviting him because it seemed like the logical next step. Rather she said it was an impulsive move brought on by the desire to be assertive.
Canon-wise, their relationship is non-existent during ToGoM and that they don't know each other well enough to even be friends. With that in mind, there was no reason for Clark to call her.
Anything between them is inferred by the author, so I'll accept that in this world, more than a couple of weeks have gone by and that they've dated a few times. In that case, Clark's a cad for not calling her.
