Welcome!!

No advice (sorry) other than time, although three months later that doesn't seem to be helping at all - just wanted to say that your boy is beautiful!! Hope things improve

He's a very handsome boy! I just love his colouring

What sort of stuff have you tried to make him more mellow with cats? Is he neutered? And have you tried feliway diffusers or anything similar?

Welcome to the forum and to Boris, he's lovely. I'm not sure what to advise about the aggression - it could be territorial, it could be that he's bored or he could just be an awkward so-and-so.

Presumably if you live in different flats, he doesn't meet the other cats very often so it sounds to me as if it's most likely to be a territory thing. If it's practical to keep him out of your mum's flat then I think that's the way to go. If it's not possible to keep them apart, then you may need to actually put them together a bit more (play with them together, feed them together etc) so that they become accustomed to each other rather than just meeting occasionally.

_________________