I think he knows. However, just for discussion sake I will comment on both sides.
We have to take into account what he could have seen in the mirror from where he was. It looks like he was more to the left rather then directly behind Alexis.
If his left eye is dominate, then he might have been able to see quite a bit of the mirror. However, if his right eye is dominate, he might not have seen a thing other then Alexis' ass.
If he did see something in the mirror, and if he really is a bit naive he might have thought the balls were a really plump pussy.
However, I have my doubts on that, since I think that would be hard given the size of those balls.
Seriously, from the cover Alexis doesn't seem to have much unlike the Griffin guy, but those balls were pretty huge for such a girly looking guy.
I've seen some big balls on girly guys in other comics, but they were all macro/inflation or just drawn by artist who just can't draw a proportionately correct set of cock and balls.