A cold and a heart murmur? I don't think they're so bad you should return him. But we rescue directly from outside, and all of our kitties required medical care at first, so "fixing" kitties we adopt or foster is part of the process to me, and a commitment we willingly take on. Knowing where he came from, I can GUARANTEE he'll get the care he needs with you, and likely was already sick and obviously wasn't getting the care he needed where he was.

URIs and internal parasites in rescue kitties - from fosters or shelters - is pretty normal. Better fosters manage to avoid these. But he came from an overcrowded foster home.
Kitty heart murmurs are odd beasts. Many kitties have a murmur just from the stress of going to the vet, but don't actually have a heart murmur. I'm assuming that's why he wants you to bring him back at some point in the future to do further tests.
Some - like the murmur I have - are just little murmurs, and don't require anything at all. I don't even need to take antibiotics before going to the dentist. Some people with murmurs do. Again, like people, some murmers require treatment so they don't progress into bigger problems. I think Eileen has experience with this, I'll PM her and ask.
My GUESS is that he was stressed and sick, and it could well be the murmur goes away.
I wouldn't return him. I'd treat him.

I would keep him separate from Calvin for at least the course of meds - you need to make sure he's A-OK first.

...and the 10 day treatment should do it, especially with zithromax.

Keep a close eye on Calvin.
And yes, a humidifier in his room will really help. If you notice he's still got the runny nose, you may want to get the bathroom really steamy and then just sit in there with him for a little while.






