Breeder male with feeder males, and keepers?

I cant really recommend anything, as I dont really know what's ailing him. A loss of weight means that he is ill, but that doesnt describe whats wrong. I'm sorry.

Keep him warm, you can put him in a hospital tank with a human heating pad on one side or top just to keep it a little bit warmer, and maybe one weaner as company so he does not get lonely. Keep him very well hydrated and dont give him any table scraps for now, just regular rat food. Call around for a vet that is used to seeing small animals, even better one that has treated rats. I had to call many offices around my area to find one.

Ok thanks! Looks like i need to go get him a friend! I am pretty sure my reg vet takes in rats. I will find out forsure!