My old man, Zylo, is also 14. He has also lost a little bit of weight over the last couple of years, but only a couple of pounds and the weight loss has been quite gradual. The vet may do what some labs call a Senior blood panel that looks for things like thyroid, diabetes, and internal organ problems.
I agree that separating him with a separate litter box is way to go, but if that doesn't work, a stool sample from any of your cats will probably do, since internal parasites that present in stool are usually transmitted to all cats sharing a litter box. So if it does turn out that the sample is positive for any parasite, you should treat all 3 cats anyway.
You've got to keep him isolated in one room w/ a litter box until he goes. There's no getting around it. It's for his own good, even though you'll probably feel bad locking him up.
I agree thyroid issues could be a problem, as well as kidney problems. With diabetes, you'd be seeing symptoms like frequent urination, drinking and eating more than usual.
Bottom line, don't freak yourself out, yet. It's much better to know what's going on early so you can treat it. You're doing the right thing by getting him checked over. Hang in there!!