Can understand your fear
BUT
All children need to learn to keep themselves safe. Abduction from a public place is exceedingly rare and the first thing an abductor would do would be to throw the phone away down a drain, put it on a passing lorry etc.
Are you teaching your son safety techniques for himself and his belongings?
Re theft, phone should be out of sight, not constantly on display in the street. Head up when walking, not constantly down, sending texts etc
Being in a group watching out for one another especially at night and on public transport
Staying downstairs on the bus, near the driver etc
Avoiding locations which are known for trouble
You cannot keep him tied to you and you can't necessarily keep him 100% safe even when he is with you.
All you can do is teach him to keep himself as safe as possible, and give him some responsibility.
It is one thing to be able to track a phone but quite another to keep tabs in the way you describe. You might well jeopardise your relationship then he may not listen to you at all.