Pre-nup. I believe if cats are involved, it's called a pre-nip.

Sign a pre-nup giving him the cats in a divorce. problem solved.

If he’s that worried about it, getting married - or not getting married - won’t solve anything. And whoever gets then should be whoever takes the most care of them. My bitch of an ex walked out and I kept the animals because I actually take care of them, never mind that she got every single one of them with minimal input from me. I’d rename them all out of spite but she actually did a pretty good job of naming them.

If you are living together how is it any different? And if you are not after 7 years..

You're trying to marry a child. Which is quite impressive considering you've been together for 7 years already.