Hi, I’m new here. I bought my daughter a bunny about 3 years ago. He hung out under the bed mostly and did well with my cats, but a year later, a co-worker gave me her bunny because she was moving. He was a year older and was also a single bunny. I had them both neutered (males) and they bonded. They hung out together sometimes but Shadow, my first one, still liked his solitude and Fuu, my newer one, was always underfoot.
Fast forward to last Tuesday, they were both fine when I fed them but an hour later, Shadow was breathing funny and looked grumpy. I picked him up and he kind of fell over in my arm, weak. I rushed him 1.5 hours to the emergency vet where he had a seizure and died before they could even get him out of the cage. 
Without thinking about it, dealing with a crying teenager, I had him cremated and brought him home. Fuu is now looking around a little, like he knows something is off. He is not as active as usual but he eats and threw his water dish around last night, so I know he isn’t ill. I think he is sad, but I also have 6 cats, all rescues, who interact with him. I really don’t want another animal to my herd, but I don’t want Fuu to mourn to death either. Do I wait and see if he perks up? If not would another male be best?
I will admit to being one of those who didn’t think when I bought her the bunny at a feed store. “It’s only $5 mom.” Now we have broken hearts and I still keep going over and over in my head wondering if there was something I missed. My room is a pesticide free zone, all cleaners are things he has been around all his life. If anyone has experience with multi pet households, I would appreciate any help on this.