So sorry to hear about your loss! Must be so difficult when ya so unprepared, obviously not something that you could ever see coming

I've only just got a pair of gliders so i've had no first hand experience with introductions. There are a lot of threads on here that i've read that have given me good advice on how to do it if I ever was to add to mine so here goes ...
Firstly, I dont think age matters too much when finding a new gliders. As long as they are roughly the same size, (the smallest one being at least 2/3rds the size of the other) then age isn't an issue. Unless you are planning to introduce a young female to your male (as you said he is stil intact) as he could possibly try to breed with her before she is mature enough (all going well with introductions of course

).
How old is your boy? I think if you did want to introduce another male you would definately have to have yours neutered first. But i've heard that after a certain age, the testosterone levels are already kind of in place in a male, and even after neutering they could still show aggressive (any un neutered behaviour perhaps) for a while. So if you were set on having another male then you should probably consider having 2 cages for a while, letting them live in the same room, see eachother through cages, possibly start pouch swapping and just go slowly to make sure the introduction went as smooth as possible.
This is the same with a female but should definately take a lot less time as your boy shouldn't feel any kind of threat or dominence test as he would with being introduced to a male.
In my opionion, you definately could do with finding a friend for your boy as no glider should be kept alone. It must be hard for him atm losing his cagemate so I wouldn't worry about rushing. Poor little guy might need some time alone to come to terms with whats happened and maybe an introduction straight away would stress him out.
I would probably go for a female though but obvioulsy one that is at least a year old and I would definately consider getting your guy neutered unless you could handle them breeding. Hope this helps. I'm sure you will get alot more advice later on today when more people are online. A few people have experienced losses recently so it may be good to talk to them and see what they're considering.
Once again, sorry for your loss. Must be hard

And welcome to the forum, I noticed this is your first post. Sorry it was such a sad one ... x