Hello, My Cat Is Approx 12 Yrs Old. Has Always

Hello, my cat is approx 12 yrs old. Has always been very active and fat. Over the last 6 months, he has been very lethargic. We have recently noticed significant weight loss, very bumpy spine, suddenly having issues with him urinateing in places other than litter box. Not sure if medical problem or just getting old. Never had a cat before...but do not want to invest tons of money due to age of cat. Indoor cat, but had him bathed and flea treatment approx 3 months ago and seems to have deteriorated since.