Oh brotha. Show me the evidence for this?
If he is replacing cats with humans, than maybe he has some issues going on. But if he can still maintain human relationships and just likes the company of cats, then no, it is not a mental illness.
I stated the last part.
Where I have worked, we've have had mutiple (many) cases where people who have had many cats and there were issues. One famous case, a woman had over 200 cats (many sticking to the floor), the house stank, she never really fed them. She was an old single lady who used to be a doctor. I've talked with other people, in my field, and they have noticed this as well with multiple cats (I set the start of it at 4).
I don't give a damn what you say, there is a issue when people go above 3 cats. I've seen too many cases, and talked to too many people, and have witnessed women (it's typically women) who start collecting cats.
Now, they may have had an issue before hand and this is just a symtom.
Some of this people still maintain human relationships, albeit, the more cats, the more reclusive they become. It's like the more the number grows, the more reclusive and or weirder they become.