1. I really don't think it's worth it getting a new rat. Your rat isn't going to live much longer, 3 years is a long time, most don't even last that long. If you get a new rat he's just going to get really stressed out, especially with his age. Introductions are stressful on young rats let alone a 3 year old. If you get another one and yours dies in the next couple months or so, then you're going to have another lone rat who will need another friend and there's another stressful introduction. I say just leave him be and spend lots of time with him. When it's his time to go he won't want a little one running around, he'll want to be alone.
2. It would probably be easier to introduce a female because they aren't so territorial and aggressive when being introduced. But yours isn't fixed and you won't find a female that is.
3. The new rat should be no younger then 6 weeks old but it's a lot easier if they aren't much older then 6 weeks because all adult males want to be the dominant one. Yours already is the dominant one because it's his cage, if you get a new one he'll want to be and they'll fight over it and like I said, that's stressful.
4. Introductions are a lot of work. I've been trying with my 2 males for about 6 months now and they WILL NOT get along (they're in separate cages). You need to quarantine the new rat for 2 weeks to make sure it doesn't have any illnesses or diseases that he can pass onto your rat. Once you do that you need to find a neutral area such as a bathtub. This means an area that neither of your rats has been in so they don't have to be territorial of it. A bathtub works great because it's enclosed. You don't want to do it in a room where they can go far or they can both get under or in something and fight and you can't get to them. When you find a place, put them together, have a spray bottle of water ready and a towel so your hands aren't exposed if you need to grab one. Let them do their thing, don't push it, if they want to walk away, let them. If they fight, spray them (make sure it's like a mist bottle). Do this everyday starting at about 5 minutes then increasing the time (only if they're getting along). When you reach about 20-30 minutes of them not fighting at all then you can put them in the regular rat play area where you normally put them. If they're fine, do this everyday for about a week. If they fight there, go back to neutral area. If they're getting along after a week, WASH everything in the cage, and rearrange everything so it's different for both rats. Put them both in and they should be fine, if not, go back to separate cages and to the play area until they can be in the cage with no fighting.
But like I said, I don't suggest it. Good luck on whatever you do.