Ideally, it would be safer to introduce another boar gradually to the existing one, on neutral territory, such as the living room floor, with towels, another ideal place would be the bath, put towels on the bottom and let them get used to each other.
When you feel that they can be put in cage together make sure that you clean the cage from top to bottom in a white vinegar and water solution, this will put them both in neutral territory.
I wouldn't advise putting vicks on the bums as this could burn and the smell could be too strong, what you could try is a buddy bath, wash them both together in the same sink with some quinea pig shampoo, that way, all their scents will be removed.
Don't under any circumstances put a rabbit in with the boar, rabbits are very powerful with their back legs and could easily kill or maim your pig.
At the minute your pig will need extra cuddles and lap time, try feeding him some apple, celery or fresh parsley whilst on your knee.
I would also advise against getting your existing boar neutered, the reason being, many vets are not used to exotics and therefore the neutering could be fatal, your pig may seem ok for a short while and the die suddenly.
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