I don't own, or ever intend to (microwave oven next to my brain?), but noticed that many seem to have an emergency dial feature, and have heard you can assign any number you want, not just 911. You should search the threads, this topic has come up before. You basically want to build a cellphone, without most of the features. It won't be cheap or simple, and you will still need a service provider, which charges you every time you dial or answer the phone, even charge you for not using your phone.
For short range, less than 100 feet, you could use a wireless doorbell, maybe re-case it. The button end is usually pretty small. You could probably add some sort of dialer for a home base. As it stands, you probably keep pretty close tabs on him in public, so 100 feet would help some.