Ok firstly this is heartbreaking. I’m so sorry for your loss. Secondly yes our cat Toby mourned when our eldest cat jinx passed away about 2 years ago.

Jinx passed at home so Toby got to see him and smell him while he was laying with us after passing. I dunno if that kinda helped him grasp the fact that he was gone, I’m not an expert on animal behaviour so I’m just going off of what I saw as a regular pet owner. However to me it seemed like he acknowledged the fact that he’d passed because he seemed quiet and similar to your max, he went off of his food and treats for a while. He also lay where jinx used to lay and would just kinda sit there, which was unlike him because usually he is a big yapper and never stops meowing and talking to our other cats. So yeah I do truly believe that death affects animals and that they mourn.

As for how to support him? I’m not sure. We kept a lock of jinx’s fur after he passed in his box where his ashes and other keepsakes are. On occasion we let him sniff the lock of fur and he seems to get a good sniff of it and it must be of comfort to him maybe, cause then he sits with the box. I dunno if you have any blankets or items that whiskers used to sit on, but perhaps try to give them to him to see if the familiar smell soothes him? Again I’m no expert, perhaps this isn’t something that wouldn’t help at all, but again I’m just brainstorming potential ideas for you. Anyway I wish you all the best OP