Oh, I'm so glad you took him in!!!!

Be very careful about germs ... speaking from experience here... as you well know . Some viruses can survive on your shoes, clothes, etc..... and be transported to your healthy cats!
The panting may have been just from thirst, but I would have him tested for Feline Leukemia .... as sometimes panting can be a sign of that.

GOOD LUCK, and keep us posted!