he'd have to get a family plan and thats obviously more than an individual phone, but im 16 and i think i deserve one. especially because im willing to pay him.
ask him, and tell him you feel like you deserve it. Give him proof that you deserve it and hopefully he will give it to you. (be mature about it too, dont whine like a child.)
Research the price of family plans with his provider compared to the plan he already has. Approach him with 3 months worth of your share of the money (probably around 60.00 if you don't have text etc), and explain why you think you're old enough, and mature enough, for a phone.