Take him to the vet.
If he isn't sick, well, you'll be reinsured.
If he is sick, it's not the end of the world. Cat are strong and resilient animals, they can suffer from illness for years and not even show a sign of pain. But the owner can help, when he notices something wrong. If my father and I had been more carefull, we could have helped the cat my father's had before it was too late. He actually had some renal disease for almost two years and only showed symptoms in the last months. And even then, if we had saw them properly he could have underwent surgery. Sadly, he was too weak when we discovered the disease.
So, act fast!
My other late cat was diagnosised with some digestive cancer. Vet gave her something like six months. She lived six years more up until her 20th year. With proper treatment, care and a loving family, cats can outlast prognosises.
So if you are worried, take him to the vet. Better safe than sorry.
If he is sick, don't necessarily put him down, there are other ways.