From all that you describe, IF that was my snake, I'd hold off on giving the Orbax; I actually agree with the reptile shop...see if the pink belly & noisy breathing are
still there after the snake sheds. Antibiotics are not without potential risks & harm: they destroy the good bacteria right along with the bad, & they can be hard on
the snakes organs & all, not to mention the risk of making resistant germs. If your pet doesn't need them, don't give them.*
You might want to see if a more experienced herp vet is in your area, if your snake does prove to be sick- as Moose84 said as well. Chances are he's OK though.
http://arav.site-ym.com/search/custom.asp?id=3661
*Don't know if you started the prescription or not...but it's important with antibiotics to give the whole course as prescribed once you start them.