One of my cats died on Good Friday & since then his brother has become very withdrawn , he just sits at the top of the cat tree all day every day he is eating & using his litter tray but he just doesn't go out & play or bother with anything 
He is usually very playful and loves been in the garden , he and his brother were thick as thieves always together and were never apart so obviously he is missing him.
I have another 3 cats ( his mum and two others) but they don't seem to be as affected by his death .
He seems to have just given up and I'm not sure what I can do to help him 