Your cat, even if he is an indoor cat could find himself outside and lost very easily.
Without proper identification, you have little chance of getting him back!
That's why it is important to get him used to wearing a collar with tags.
Most cats, especially older ones, don't like to wear a collar and especially one with clanky cat tags hanging from it. To get him used to it, put the collar on gently and let him wear it for an hour or even less if he is really cranky about it. Gradually work your way up to longer wear times. You can try giving him a treat or favorite food after you put on the collar so he will start looking forward to it. Eventually your goal is to get him to wear the collar at all times.
When purchasing a collar and cat tags, make sure the collar is sturdy with a break away feature so that your cat does not strangle himself if he gets caught up on a branch.
The tags come in all shapes and sizes - I would pick the smallest one that can hold your vital information. Make sure you have it engraved with your address and phone number where they can leave a message if you are not available.
Thinking about getting a microchip instead of cat tags? Read this fascinating story about a cat that was returned to it's owner after 10 years.