Advice?

My rat Ky(almost two) just lost his brother, and he's been pretty depressed with worsening physical health. His brother had respiratory issues on and off and just suddenly passed away at home. Ky still has four other cage mates he gets along well enough with, but when they come in pairs they're always hit really hard when the other dies. Does anyone have any advice? Is there anything I can do to help him?

Note: I made sure they had all seen the body with time to register it, and I've been trying to encourage him with treats and so on but he's been really faltering. Has anyones rat had their main friend pass away and recovered?