Is it ok to keep a lone, elderly rat?

I have a male pair of rats (2.5 years) but one of them has been aging more than than the other. He's doing really bad. I'm actually at the vet now with him and I don't have high hopes...

I foresaw this and was preparing to get a new rat for weeks now but I can't find an older rat and introducing a baby to male rats sounds stressful based on my research. Right now I am pretty heartbroken over my sick rat so... I don't know if I can deal with getting another rat.

I want to keep my surviving rat and am willing to spend 2-3+ hrs daily with him for the rest of his life... but if it's inhumane I can also rehome him even though, for me, that is the worst case scenario and I worry that will be such an adjustment for him at the end of his life.

Any advice? What would you do?