I need help!!!!! 4-5 week old kitten URI or Felv?

I saved a kitten that was 4-5 weeks old, I was told he may have a URI, I was given Clavamox drops to give him twice a day for 7 days. He has a slight whistle/wheezing sound when he breathes. He has no crust around eyes or nose. He eats and drinks with no problem. Uses the bathroom, it all seems fine, he is on the skinny side a little bit, But isn't that to be expected on a kitten found on the street? I don't have much money, just very worried I have a 15 week old kitten also at home and want to make sure my other kitten can't get sick. But also want to know if you think its just a slight URI or is it serious and he may have FelV?