Well,

Instead of picking him up and stressing him more, I put my hand in front of him and listened to his back with my stethoscope. It took a minute to get situated, but after a minute or so I did hear what I assume was normal breath sounds. Definitely no crackling or popping, just a very quietsoft airy sound. He didn't really appreciate the cold stethoscope, but he didn't attempt to run away. Just flattened down as much as possible and tried to be invisible. That's his usual behavior.

Since he isn't wild caught, how would he contract any sort of infection? I guess he could get a respiratory infection from stress, but not chytridiomycosis, right? We're very careful about hygiene.