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I apologize for the wait.
With any FIV patient it is really important to be on top of any illness and it certainly sounds like you are. I think at this point you should take him the vet ASAP. Limp with shallow breathing could be quite serious even a cat that does not have FIV. Depending on how he looks I think they are going to want to run bloodwork and chest x-rays to see what is going on.