I agree, he definitely needs a partner (and of course has to be neutered before). I had a few rabbits that were kept alone before, their owners gave them away because they were extremely aggressive, but most of them changed after introduction.
I would recommend to let them stay outside in that tortoise pen (it looks predatorproof?) or build another big enclosure, but please don't try to put two rabbits in a small cage, they could hurt each other.
This is large? The animal shelters here would never give a rabbit to someone with a cage smaller than 40 square feet. They need lots of space to run and jump and get out of each others way, because they can be quite rough but nevertheless need (at least one) partner.the cage is very large. About 22"x12"x12".
I would recommend to let them stay outside in that tortoise pen (it looks predatorproof?) or build another big enclosure, but please don't try to put two rabbits in a small cage, they could hurt each other.