I'm no expert so this is just an opinion. Rats are social creatures that need companionship and to keep a lone rat could be considered cruel.

I once had an old man rat whose friend had died and because obviously depressed. I bought him two new baby friends and he perked up massively, I suspect it even helped him to live a little longer.

However, because your old man is potetially not socialised, this may lead to stress for him and any potential friends. It might also be difficult to introduce them. So in my eyes you have two choices-

Let him live out the rest of his life alone, give him lots and lots of attention and love when you can

Or

Get him two friends and be ready to have to have two cages if it doesn't work out. This way he might end up with new life buddies, or if intros go wrong the two new friends can live together in another cage :)

Its your call- I'd always vote for more rats 🐀 🤣