This is Dr.Christian.
Thank you for the information. Certainly his weight is worsening any underlying problem he has. There are several causes for panting and shaking:
In this case I think we can rule out nausea, most medical conditions have been ruled out by bloodwork, and nervousness is probably out as well. That leaves pain, antibiotic reaction or a respiratory problem. Most antibiotic reactions will occur within the first few days so that may not be the cause. I'm concerned that there may be a respiratory problem going on and a recheck by your vet may be necessary to find the cause. Antibiotics should have improved the situation if an infection was the cause.