Might look into a wireless doorbell, usually under $10.00, shop around, some have some fairly small transmitters. Mine uses a 12v battery, about the size of a AAA battery. You would need to add a circuit to pulse the transmitter in place of the button, maybe a 555 timer could handle this. On the receiver end, you would just need to detect when you miss the pulse, another 555 circuit might work.
I haven't had great luck with RF circuits built from scratch, very little range, drift, and interference. Seems like kind of touchy stuff, placement of parts is sometimes important. Just so much easier to use something ready made, and add to it.
Cats don't generally like collars, or anything attached. Might have some trouble with that. He might slip out of the collar, or strangle himself trying to remove it. Cats are pretty quick, if he wants to wander there isn't much you can do. Hate to suggest it, but with male cats it's a good idea... get his nuts clipped (ouch)... Not much reason to wander, if you provide food and water, too embarrassing to seek a mate. They also don't spray piss (musk) all over everything, which I'm sure has cost more than a few their lives. Not only does it stink, but its corrosive, nobody appreciates it.
Basically, you are going to outlive quite a few pets in your lifetime, no matter how much work and money you sink into prolonging your time together. Cats are great, they rarely die at home, if they can avoid it. They leave you with a mystery, their end is left to your imagination.