He has times when he chases her around and mounts her, multiple times and she is whinging/crying. I don't like it. I had been thinking of getting another girl for solidarity purposes but I don't know if I could put a baby through it all.
I'm guessing your female pig will be going into season and your boar (though he's neutered still thinks like an entire) will take full advantage.
Most female pigs will tell an unwanted advances to "Do one" when they get fed up.
Instead of a baby pig, can you look for an adult female to join your cavy group. If you phone a rescue they can match up. They'd need to know the full history and pick the right pig for you otherwise you could end up with two pigs against one and potentially a bullying situation. But your boar might enjoy being the head of a hareem (if he can keep his mucky paws to himself)
They will have heard the situation before.
There was a post a while back (IIRC the 'Wedding' was last Easter. The OP had a single boar that she had neutered. They found a bolshy female who didn't get on with other females and matched her with the boar.She was a strong enough character to fend him off) . I can't remember the OP name but the female pig was called Beyonce. Might give you some ideas.