Thanks to everyone for the good thoughts. It seems like they helped. After the vet shaved Logan's neck in preparation for the biopsy, he noticed that the bump had become significantly smaller. He called me and we decided to wait a couple of weeks on the biopsy and see whether the bump disappeared on its own. Logan is going to be taking benadryl twice a day and we'll just see how he does.
I suppose we could have done it anyway - it wasn't the money - but I don't want to put him through anything he doesn't need. He's afraid of the vet as it is. So we'll just wait and see a bit. Maybe it was a big bug bite.
Again, thanks all!