You don't deter it.
Let him go outside. Buy a leash. Let him get used to being outside.Â
I mean if you want to keep him as an indoor cat, that's fine. However, if he is very easily able to get out then give him at least a little bit of time outdoors so he can get used to it.
The thing about indoor cats and being outside is...when they do manage to get outside they go into panic mode because everything is just so different. That's how you end up having cats that run away because indoor cats don't know what the hell they are doing if they manage to get out.
Make sure he is neutered too. That will help.
So tl;dr
Slowly allow him to get used to outdoors so that if he manages to get out, he won't be completely overwhelmed and run away. And remember, if he does run away, keep in mind that cats who run away get very scared and won't run away further than a 3 house radius.
Let him go outside. Buy a leash. Let him get used to being outside.Â
I mean if you want to keep him as an indoor cat, that's fine. However, if he is very easily able to get out then give him at least a little bit of time outdoors so he can get used to it.
The thing about indoor cats and being outside is...when they do manage to get outside they go into panic mode because everything is just so different. That's how you end up having cats that run away because indoor cats don't know what the hell they are doing if they manage to get out.
Make sure he is neutered too. That will help.
So tl;dr
Slowly allow him to get used to outdoors so that if he manages to get out, he won't be completely overwhelmed and run away. And remember, if he does run away, keep in mind that cats who run away get very scared and won't run away further than a 3 house radius.