I have tried keeping boars together, boars alone and neutered boars with sows and had very mixed results. In my opinion, boars can be very difficult to pair without fighting, even once they are castrated as they tend to make a nuisance of themselves with sows. I will say though that they are definitely happier with company, even if it does mean putting up with the odd bit of aggression. My neutered boar now lives with 2 sows and I have only just realised how much happier he is. He made SUCH a wonderful pet by himself because he was so tame and attention-seeking but I now realise a lot of his behavior such as bar-chewing and constant squeaking was boredom/frustration behaviour caused by him being alone. Now he has his sows he seems much more content. you could try introducing another boar but everytime I have done this, even with brothers kept together from an early age, there has been some degree of fighting, sometimes minor enough to carry on keeping them together, sometimes too bad. a divider might be the best option?