Getting him another boar in that close of a proximity of sows could be a huge problem. They would fight and blood would most likely be shed. I have 2 neutered boars that live in with sows, however they are living in seperate cages so there is no interaction between the boars. They have their sows and are content.
IF you can find a cavy knowledgable vet that has neutered guinea pigs with success I would try that route. If you can not find a vet with any knowledge he will have to be content watching the sows between the bars.
During the day I would take his wee lil house out so he does have to interact. My herd has no houses, they however have ledges to snuggle under if they need safety. If they did have houses I would take them out during the day so they could interact with me.
I hope that helps... Good luck!