Bee, this is very sad news, and an awful shock, I'm so sorry. Poor Drum.
Pursley went very quiet and clingy for around 10 days after her pal Chubby passed on. She was never a cuddly cat but seemd to draw comfort from sleeping between her humans at night and curling up behind her Dad's head on the sofa in the evenings. She's getting better now, but has adopted Chubby's old sleeping place (which I've left un-cleaned so his scent is still there for her) and she seems to find comfort in this.
In another life, one of our young cats was run over on our driveweay by a neighbour. Her brother pined for days and we too brought another cat home to try to soothe his pain, but it was a bad idea and he hated the new kitten (although I was ignorant of correct introduction methods back then).
It took months before they would sit together in the same room and I regretted it for his sake as he just wanted his sister back.
I hope you are able to find some comfort in each other, and in time Drum will learn to be happy again with your help.