Jasper is possibly getting better.
He doesn't have a temperature, nor fleas, nor Unexplainable Bumps. He had a slow-release antibiotic injection, decided that (today) he'd eat 3 Defurrum medicine-disguised-as-treats, and contrived to start moulting handfuls the moment he was put on the examination table, then to stop again on the way home. If he hasn't eaten (not counting the Defurrums) by tomorrow evening, I'm to take him back.
Slightly less worried now. Thank you for all the thoughts & comments.