I can only agree - he had a fight with his last cage mate because they are incompatible not because he prefers to be alone. He needs to find the right, compatible friend to be able to share territory with.
A Comprehensive Guide to Guinea Pig Boars
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
However it is important to note that the cage you have is too small if you get him a new boar friend.
A single piggy should have a minimum of 8 square feet of cage space. So the cage you have comes in a little too small even though he is on his own at present.
If he has a new boar friend, then you need to provide a cage which covers 12 square feet
for two boars. Please don’t try to keep two boars in the cage you have as lack of space will cause problems between them and potentially a fight.
If he is neutered and has a sow friend, then the cage should be 10 square feet.
Cage Size Guide
I know neutering is expensive in the US and it can be harder to find a knowledgable vet, so if he can’t be neutered then he obviously can’t have a sow as a new friend.
Sows are usually only spayed for medical purposes as it is a major and invasive operation - you may never find a spayed sow (they certainly do not come into rescues in the UK at all often).
Please do get a safe playpen if he is going to go outdoors for any supervised time, and don’t use the harness anymore.
Harnesses are incredibly dangerous for their delicate spines and can cause the death of a piggy.
Potentially Dangerous Cage Accessories And Toys
I would also recommend that you remove the hay rack which is on the outside of the cage and instead just put the hay on the floor inside the cage in a big pile. Hay is of course their main food source but it is also a great source of fun and enrichment. Playing in hay and being able to forage in it, make little nests and sleep inside it, is all great fun for them. Particularly while he is alone offering him lots of ways to use his natural instincts to keep occupied is important.
Another idea is to not use food bowls and instead throw the recommended portions of veg (one cup per pig per day) and pellets (just one tablespoon per pig per day) into a pile of hay so it gets mixed in and piggy has to search for it. It makes meal time much more interesting and makes them use their brains!
Long Term Balanced General And Special Needs Guinea Pig Diets
He may not be coming out much for two reasons - he is too unsure at present, you’ve only had him a few days and he needs to settle in; and because there isn’t much for him to do inside the cage (a friend is the best form of entertainment though). Cuddly toys don’t offer anything for guinea pigs.
The guide below details enrichment ideas.
As mentioned above, if you can’t find a compatible friend for him to bond with, the next best thing is living side by side with another piggy so they can interact through the bars of their cages but don’t have to share their territory.
Enrichment Ideas for Guinea Pigs
You will see I have added green links for further information throughout the post.
I have also added the link to our collection of new owner guides below. There is a lot of them there but detail everything further - housing, diet, as well as other care information such as the importance of weekly weight checks so you can monitor that he is eating enough hay.
New Owners' Essential Information and Practical Tips Starter Collection