Have an indoor cat approximately 16 years old, and he has suddenly been very fussy about what he eats.?

His name is Mister and he's been a great pet but is acting peculiar lately and is also losing weight. I have been supplementing his diet with a tube that has an essential mix of vitamins and stuff already in it. Also - he gets crazy with water and drinks what seems an awful lot. My Mom thinks he has intestinal parasites that are out of control. Never see nothing out of the ordinary though from that end, but he is also throwing up more frequently. I haven't the money for a vet, but will find a way if someone thinks it serious enough.