Clindamycin is one of the newer ABX. I have NO clue whether or not it would be safe for cats.
Ok, did a little research. Clindamycin is used mostly on serious bone and "soft tissue" infections- that would be cellulitis. It *might* work on upper respiratory infections, but I suspect there are cheaper, safer and older ABX they use instead- mostly because overuse of any of the newer meds just speeds up the whole resistance problem. The dose is 5-15 mgs per pound of body weight ONCE daily.
If he's eating fine and his lungs seem clear (when his nose is cleaned off so he can breathe, he's active, doesn't use his stomach muscles to visibly assist his breathing and you can't hear wheezing), you may simply be able to provide supportive treatment- nutritious food, cleaning his eyes and nose as needed, making sure he has plenty of fresh water, and watching for worsening symptoms- and see how he does.
MOST of these illnesses in kittens are viral in origin, but can then turn into secondary bacterial problems which can be fatal. For that matter, some of the viral illnesses can be fatal, which is the reason I strongly suggest vaccinating healthy kittens- they may have nine lives, but there are so many nasty diseases they can be exposed to, especially if they are outside cats, that vaccination gives them half a chance. Plus, some of the disease problems are incurable and eventually fatal- including contagious feline leukemia. Modern vet medicine can extend their lives, but can't cure them, and they often suffer pretty badly before the owner decides they aren't ever going to improve.
Summerthyme
