Sarah,
If you have one of those cats who just has to go outside, like mine do, I would make sure you have a collar and good identification on the collar. You might also consider a microchip, which is more common in the US than a tattoo, I think. But I have heard of those who do both. You want to do what is most common in your area, to ensure that whoever eventually would find him would hopefully be able to identify him that way too, in case the collar got lost.
Neither of my cats have a microchip, but both wear id on their collars. I am lucky because mine stay close to home always. Mimi is older and rarely goes outdoors, and Butter knows he has it too good to wander far! LOL!
Other than keeping him in...which I know he doesn't want, I don't have any more suggestions. Good luck.

Logan