Bellpepper is now about 3 - 4 years old. Is it possible that he might have learned some social cues after living with Abby for 2.5 years? Is it worth trying
to find him a male friend? (We've scoured the rescues, shelters, exotics vets, etc looking for a spade female and there are none).
The problem isn't an understanding of social cues. It's finding a pig with the right personality, one that will mesh with your boar. Guinea pigs are a lot like people that way. They can be picky about who they decide to be friends with. If someone picked a friend for YOU and the person they picked was very different from you in every way, the likelihood of the two of you being friends may be slim. You can't force someone to like you or force yourself to like someone else if you have nothing in common. Guinea pigs are just like that. Obviously, your boar liked Abby and had no problem getting along with her.
Make sure your cage meets the size requirements for two males before you take him to find another buddy.
He adored Abby, but still had times where he chased and mounted her for hours, and we would sometimes feel the need to divide the cage for a day or two. That behavior lessened quite a bit after the first year together.
Our cage is 2 x 4 grids. And we can easily expand.
But you say there is a fair chance of finding a male friend at this older age?
If you do get a baby male, remember that he too will go through a snotty teenager phase that could last from about 6 months to 18 months of age. Bellpepper may fight back and you'll have spats during that time. During that time they will need a lot more space, probably more than a 2 x 5 if it's at all possible. Maybe also have a divider you could place in temporarily should the fighting get bad. But things should get better.
Our snottiest pig out of 4 babies we had is now at 3 years of age one of the most mellow ones we have. I never would have guessed that while we were in the middle of the snottiness (she was snotty toward people too, lol), but it's undeniable now.
Just saying, that it does depend on the personality. I put Scatter first with another neutered male, and it was definitely a no-go. Then I put him in with a dominant female and they became comfortable, if not super bonded. After she passed, I tried Fuzzbutt. What a match made in heaven! At that time Scatter was 4 and Fuzz 3.5 years old.
They're now in their scrubbed out cage, pretty quiet and calm. The little baby is so afraid of me that he runs right over to Bellpepper for protection. And I think I saw Bellpepper lick the baby's face - something I never saw him do with Abby (not that she would have let him). I hope these two become good friends.