Goldfish is being chased and won't eat?

My son has 3 goldfish and one died on Friday. For the last 2 days one of the remaining 2 goldfish keeps chasing the other one, especially when I'm feeding them. He's quite aggressive & pushes him alot around his gills. I feel so sorry for him & I'm not sure what I should do. Now he's stopped eating also. Is he sick or stressed from being bullied?