Blessings of the day;
Our cat Calamity Jane Grey had a stroke at age 21 years 3 1/2 months, but continued to try and function normally. Her appetite stayed good and she tried to maintain good litter box habits. She did not seem to us to be having any pain.
Symptoms she had: body bent to the left so she veered a bit as she walked (but kept a shoulder near a wall to guide herself), she did drool a little more, her eyes didn't focus quite as well...
The 3 younger cats all were still deferential to her as Boss of the House and seemed to snuggle with her more.
We got her to our vet, who after a thorough examination, prescribed an enzyme treatment with which he had had success on other feline stroke victims.
It seemed to be helping our old friend, her body was straightening and she was walking better.
Unfortunately, Cammie had another stroke a couple of weeks later. I held her head up while she ate one last time. She was still fighting for life, but was too weakened by a body that failed her spirit...she died with dignity when we brought her to the vet for the Final Grace.
Since your Tomcat is younger than my Cammie was, maybe such a course of enzyme therepy will work better. See your vet, please.
good luck - Pip
Have your cat checked out for Hyperthyroidism and feline diabetes. They're fairly common illnesses in a cat of that age...and both would fit the symptoms you describe.