Possible scent aggression ?

Hello, I have 4 cats, all of which are blood related, however the youngest one who is disabled seems to be hissed and growled at constantly when he goes near the other 3. To give context, I moved out of my mum's house over a year ago with 4 of the cat (we had 7). My house isn't has large as my mum's (but still big enough so all 4 have space from eachother) so they are in closer proximity to eachother than they used to be. However the 3 older girls seem to growl and hiss at the younger boy and I can't figure out why. My only thought would be scent aggression, as I have caught them sniffing him and then hissing, but they've known him his whole life. Is there something weird about his smell?? Is it because he is disabled? Any advice or tips? They are all spayed, we give them equal amounts of attention, have enough food for all of them etc. I have tried one of those relaxing cat diffusers, but I dont notice a difference really.