Welcome and I'm sorry you are begining posting with concerns for your kitty.
First, I don't think 8 is 'old' for a cat. It does sound like something serious is going on, and I couldn't even venture a guess as most of my ideas are probably very scary (and hopefully way off). Ordinarily with a limp, I'd just think a pulled muscle, but the labored breathing and decrease in activity are all signs something more serious is going on. Lets just say it is a good thing you'll be taking him to the vet Wednesday.
I'm sure they'll do an exam and want to pull blood to see what is going on inside with him. Please let us know what the vet finds, as your experience may help another cat owner with a similar problem.