If he's anything like my Dad, I'd try to convince him that you know how much work having your own dog will be(especially since some shelter dogs tend to come with issues that require persistent training to remedy), and that you're ready for the responsibility. And let him know that the dog will be YOURS, you will give it all the exercise, training, and everything else the dog will need.
And already know, before going into this, what traits are important to you in the dog, that will show you're Dad that you're serious about this.
And just remember, a dog is a lifelong responsibility, don't just think about where you will be next year...think about the future. Will you get married? Have kids? Get a job where you can't properly excersize and train your dog evey day? What of you can't find good work and can't afford the dog? What about then? You need to know you're ready and have the money to keep the dog, and be confident that you'll be able to handle anything that comes with your dog in mine
I was just throwing that out there because I know several people who were around your age when they got their dog, and wish they had considered the future more 
Good luck!
~IJMB
And already know, before going into this, what traits are important to you in the dog, that will show you're Dad that you're serious about this.
And just remember, a dog is a lifelong responsibility, don't just think about where you will be next year...think about the future. Will you get married? Have kids? Get a job where you can't properly excersize and train your dog evey day? What of you can't find good work and can't afford the dog? What about then? You need to know you're ready and have the money to keep the dog, and be confident that you'll be able to handle anything that comes with your dog in mine
Good luck!
~IJMB