Thanks, Steve! Fingers crossed. He's to see the vet the day after tomorrow.
Incidentally, I was advised by a herbalist friend to take vitamin C and Vitamin D3 when I had a tough chest infection that wouldn't give in to antibiotics. Half an hour after starting taking them, the infection suddenly cleared - which may of course be a coincidence.
The doctor told me to be careful of Vitamin D and not to take too much, because it's one of the few vitamins you can overdose on. I've accordingly taken it but in careful doses since.