Quarantine: If he is really struggling being alone then you may decide to not quarantine and instead put his cage directly next to the girls.
If you do quarantine then Ideally it is in a different room entirely. If you cannot do that then keep him as far away as possible for the quarantine period.
Then, post quarantine or if you decide not to quarantine, you will then need to move his cage directly next to your girls cage for them to interact (with security measures - lidded cage or c&c cage two grids high at the divide just to be sure) between the bars.
Regarding neutering and bonding.
He is on the older side to be neutered. It’s not totally out of the question but you will want a knowledgeable and experienced vet and to be sure he is as healthy as he can be going into anaesthesia.
However the other thing to consider is your girls are past ideal pup bearing age, and sows once they are older than about 3 can be less willing to accept a boar at all. It does come down to compatibility so nobody can say whether they would or wouldn’t accept him but it is a lot to consider given everybody’s ages.
In this situation it would be perfectly feasible to decide to just keep him side by side permanently.