I suggest that you use a contract system. I did this with my son when he was about your son's age. True, it wasn't about a phone, but a complicated and expensive calculator. The first one that I bought at the start of the school system disappeared pretty soon afterwards. The second one was dropped and broken.
So we sat down for a conversation. I made sure I was calm. First, I reviewed the costs of the calculators, making sure he understood how much the two lost/broken ones had cost. You might do the same with the phones, going over the prices, the monthly access fees, and any related costs on your phone bill.
Then I told my son I would buy one more calculator. If at the end of the school year he still had it, in excellent working condition, I would rip up the bill. However, if at any time during the year he lost/broke/damaged/loaned it, he would be responsible for the entire cost, plus any replacement costs. He would not get a new one until the entire cost was paid back in full.
We put this down in writing, and I reminded him that soon he would be getting jobs, and expected to perhaps sign a written agreement with his employer, or be responsible for company equipment, or have certain standards in writing. It was time he learned to be responsible - in writing.
This way, your son has to agree to use a sturdy case, not treat the phone carelessly, not loan it out, not go over data, etc. If he still has the phone in excellent condition after a period of time that you establish (for example, one year), you won't ask for the cost of the newest phone back. Or you can tell him that he has to keep the phone for 2 years which is most cell phone providers length of the contract. Whatever works for you. And if after a year or two he wants an upgrade, he'll be responsible for that cost too.
But put it in writing, and sign it - both of you. Keep the contract where it's safe and accessible. And don't treat it as a harsh punishment, but as part of being an older teen, part of being more responsible. Remind him that being responsible and reliable will be part of getting a learner's permit, a driver's license, a job. This is his amazing chance to demonstrate that he can be responsible. Make it a goal, a chance to earn the privileges that come with being 16 and older. Keep it positive.