If you just put him in their cage without doing neutral territory introductions then they will see him as an invader.
It may simply be that they don’t want another piggy in their pair.
He must not be having floor time with the other guinea pigs. Any piggies who are not bonded must not have physical, contact. Each time you put them in the same space together it is a bonding session, they start to set up their relationship and then you separate them and that results in stress. Then when you put them back together they start the process all over again. Piggies do not do these kind of play dates. Try they have to be permanently bonded or permanently kept apart.
I note you say you are trying to get him another boar. That would not be a good idea. As he is neutered, he will need to live with another sow.
As you already have sows in the house and your boar has had access to the sows, he will not bond with another boar. The smell of sows alone will cause fights between two boars in who live together. The fact that your boar has been around sows and had physical contact with them will likely prevent him from accepting another boar in any event