It’s fine for him to be kept alongside the others for interaction through the bars and await his neuter. However, it is months be he can be neutered plus have his six week wait afterwards so it will be a long time before he can go back in with them . The decision has to be yours and what you can handle but -
- is your cage big enough for four piggies - that is 210cm x 60cm.
(if you separate him put and bond him with another boar, then the cage for the three sows needs to be 180cm x 60cm)
- are you able to manage five piggies? If not, then awaiting his neuter and bonding him back with your other piggies is probably best
- if you get him a new friend of his own , then the best way is to take him dating at a rescue centre. That way he can choose his own new character compatible friend. it may be that he actually would be happier with an older boar.
If you buy a young boar from a pet shop, then they will be absolutely fine together while they are young but you need to be prepared for the fact that when they grow and hit their teens (which is from 4 to 14 months of age) then there is a risk character incompatibilities will come out. If that happens then they will need to be separated and live in separate side by side cages. More boar pairs make it together than not but when you are dealing with two youngsters for whom you don’t know how dominant they are going to want to be, then you have to have a plan b in case it goes wrong.
If you do get another boar for him, then their cage needs to be 180cm x 60cm. Boar pairs need more room as they are territorial. The guide below details information on boar specific behaviour and care - respecting their hierarchy, two exit hides only, multiple items etc
You will also need to keep a boar pair away from the sows. The smell of sows around a bonded pair of boars can cause the boys to fight over simply smelling the girls. It’s therefore advisable to keep boar pairs in a separate room to any sows. You must always handle the boars first so you don’t transfer girly smells on to them. If they have to be in the same room, then you will need to keep them away from sow pheromones and that is best done by using stacking cages with the sows at the bottom so no smells Can fall downwards onto boys.
The guides below will help you further
A Comprehensive Guide to Guinea Pig Boars
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
Dominance Behaviours In Guinea Pigs
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Neutered / De-sexed Boars And Neutering Operations: Myths, Facts and Post-op Care