i am practical when it comes to pets. that's why i would have him evaluated by a vet, but i would NOT be talked into any expensive tests or treatments. no way. i would set a budget for the visit (under $100) and see what they come up with. they should be able to give a thorough physical exam and run blood tests for that amount of money.
that said, i have had most of my pets die naturally, at old ages. recently, our 13y 7 mo golden retriever died outside, in the late morning, in june, under a pine tree close to the house. she had eaten cat food just 1-2 hours earlier. she had her annual exam at the vet 10 days earlier, and he noted that she was slowing down, but otherwise not with any noticeable conditions. hers was a good death.
i lost a cat, age 19y 7mo. there was something in her cry that made me aware that she was not herself anymore. i took her to the vet, who noted she was down to 4 lbs, 2 oz and definitely slowing down, but, again, nothing noticeable/treatable (as if you can even treat conditions in that advanced of old age). she slowed down completely, stopped eating and died stretched across the heater vent in the bathroom within about a day and a half of the vet visit. another pretty good death.
earlier i lost another 19 yo cat, who got progressively skinnier, and slowed down his activity. on the morning that he eventually died later in the afternoon, he came to the kitchen at feeding time, watched me open the can, then turned around and walked out. he spent his last morning sleeping in the sunshine on a living room chair. then he sloooowwwly walked to the bedroom heater vent (it was january) and laid there, barely moving, barely breathing. i decided to have him put to sleep. my mom came over to help me transport him to the vet. in his fragile condition, i decided i would just hold him on my lap in my car. while my mom backed my car down my driveway, i felt a wet spot suddenly appear on my jeans leg. he has passed away in my arms (and thus drained his bladder). excellent death, don't you think?
but i have also had it go bad. i had a 13 yo cat, whose first sign of something wrong was a messed up eye. turned out he had cancer. he passed within 2 weeks of being healthy, to going fast. he was overweight, and died so soon, he never got skinny at all. i had him to the vet the night before he died, and the vet gave him a shot of cortisone, said it was 50/50 that it might work and he could live another 6 months, or it would go the other way and he would die soon. it went the bad way. by the next morning, he could not move. it was pitiful. he cried in pain. but i was terrified to move him as any movement on his own elicited a sharp cry. he was in agony. and i felt horrible that i had not had him put to sleep when i had the chance the day before. he finally passed after about 2 hours of agony. towards the end, i heard a *pop* noise from within him, then he lapsed into a coma and never came out. i knew he was gone when he stopped breathing. a very poor way of passing. i regret it a lot.
sooooo.......... my bottom line recommendation is to get some professional input. that way you have some guidance. and if things do go badly, you can have some solid advice on how to proceed.
wishing you all the best!