Hi. So sorry for your loss. Its very sad when we lose one of our rattie friends. I agree with the last poster that at his age, I don't think getting him another cage mate is a good idea. The truth is, he's an old boy himself now, and if you do end up getting another rat, you could well end up with another single boy on your hands after your current boy passes away. He will miss his cage mate but in all honesty, I would handle him all the time yourself now, giving him lots of love and as much company as you possibly can. He will be ok with just you, his loving owner for company for however long he has left. Give him lots of cuddles and attention and whenever you see him pottering around, bring him out to you and let him know he's not totally on his own.
Yes, ideally, another rat would be great, but if he is territorial it could cause him more stress than being by himself and that wouldn't be good for him. Plus, you said you couldn't afford another rattie. Yes, you could get another for relatively little money but when your boy's time comes to pass, the new one will be left on his own - a never ending circle really. And if you cannot afford another rattie, then the new one could bring all sorts of problems with him, vets fees etc. Not very practical at all.
Just be with him as much as you can. Show him lots of love, play with him as much as possible and let him live out the rest of his life with you for however long that is. He will be ok, and so will you. I promise. Lots of love to you.