I adopted a 4 year old who had lived alone her whole life. I tried her with 3 different pigs and she drew blood on all of them. I think she got too used to "running the place" and didn't want a friend. It doesn't mean your pig will have the same experience.
An ideal situation would be if you could find a rescue in your area who would allow you to bring your pig over and meet another to see if they get along. That way you could be sure it would work out before committing.
I didn't say shelter, I said rescue. A guinea pig rescue would not introduce another pig unless the pig had undergone its quarantine at the rescue already and treated for mites if needed. It is a somewhat common practice for rescues to encourage the potential adopter to bring their current pig in order to find the best match.
Rescues quarantine for the adopters prior to introductions, so there would not be a risk of picking up illnesses. We've had a pretty high success rate of pigs who have been stuck alone for years being introduced to new pigs.
We find a lot of times that it can take a bit of extra time for them to figure things out.... it's almost like their years of being isolated have made their social manners a little rusty.
I have had several older boars that I have introduced a younger boar with. All has worked out very well except for one peruivan boar who is a mean ole thing and has not let anyone live with him. But the funny thing is he loves the other boars next to him that he can see and smell thru the C & C cages.
I would see if a rescue near you has a younger boar. Good luck.