A couple of ideas.
Some cats just like the feeling of laying on grass. You could try getting him a bed of grass in the house. If he still persists, get a cat/small dog harness for him and take it a priority to take him out before breakfast, afternoon, and before dinner. that way he's never out of your site. I know a lot of friends that do this. It is much safer as well. If you want to get even more strict, you can get a animal stroller and take him on walks.
I think truthfully the harness will satisfy the urge to go out while still being protected. And that way you can direct him to where she should be going.