I've had just about every dog in the house on Tylan at one time or another, and all are on it at the moment (long story). It doesn't seem to suppress appetite. Most won't eat as a powder on their food, so I use capsules or tablets as available.
I'd suspect some other issue--perhaps the underlying infection, or as FiveRoooooers said, a bit of residue on the capsules, or a food problem. I hope the new meds the vet prescribed, or some of the suggestions others have posted, help.
Congratulations on your new boy. He's very handsome. I hope we'll soon see pics of him all better.