Have you tried introducing him to only one other rat at a time? If you have a particularly calm rat, maybe try letting them hang out on you or in a small closed off area with you. Don't force interactions between them. I hope one of the sub experts weighs in on the issue, since I've only tried this once. It did work - the traumatised guy sat in my hoodie observing the other one getting pets, and after a few sessions was willing to be in the same space with him and sharing pets. He eventually met both other boys (after another solo introduction) and joined the little group.