Thanks for writing in,
Has he had any vomiting?
Is he an indoor or outdoor cat?
I am definitely glad to hear that he is feeling a bit better today and not vomiting. There are many things that could be going on with your old boy. The first thing I always think about in an older cat is kidney disease and hyperthyroidism. Kidney disease will make a cat nauseated, hence the drooling, and the can also loose alot of weight. Generally they don't seem to feel very good though, and may not want to eat. A possiblity though.
Hyperthyroidism is again very common in cats. These guys generally eat alot and still lose weight. So what I would suggest you do is bring him to your vet possibly tomorrow if this isn't something that resolves itself in the next day or two. They will want to do a full physical exam on him as well as some labwork to rule out such things as Diabetes, hyperthyroidism, and kidney disease amoung others. I hope this helps and I hope your boy feels better soon! Let me know if you have any further questions.
Its very possible, or it could be one of those things that have been there in your cat but are just now showing itself. Cats don't generally get colds like we do. If they do its usually an upper respiratory type infection so you would see sneezing, nasal discharge etc. But just like us sometimes they have days where they just may not feel that great. SO therefore, if he doesn't feel like his old self in a few days then you know its likely something more and should be seen then.