Has he been neutered yet?

Unneutered males do have a more agressive and territorial instinct in them. He might hear the tone of voice as a threat - and who knows what he went through and learned about louder voices before he arrived at the shelter?

Chat with your vet, and if money is a problem, ask if there are spay-neuter funds around. Even on Google, you can type in "free spay and neuter in ****(name of province or state)" and get some resources.

Good for you for rescuing this guy! I am sure he appreciates your nursing him to health after that nasty URI - now his hormones might be acting up.

hugs
Catty1