Urgent help needed for my mouse!?

My mouse was in a tank with 23 other mice that I rescued all from the same tank but he was the smallest so got really badly bitten all over his back. I took him out and he is on his own and his hair is falling out, his skin looks sticky, slightly yellow in some places and patchy in colour and lumpy. He can't use his back legs too well and his fur is scruffy and not very well groomed. He also has dihorroea. What can I do to save him I thought he was healing but it's gone so bad. What should I do?