This doesn't have to be a line-in-the-sand, my-way-or-the-highway sort of deal.
Being snotty is a job requirement for 15 year olds. They're hormonal, they're too old for kid stuff and too young for adult stuff. They don't want to be driven everywhere by parents, but they can't drive themselves. They hate the world. They're preparing - slowly and poorly sometimes - for independent life at 18, which means creating some distance between themselves and their parents.
So, I'd separate the issues. Issue 1 is birthday and financial constraints. Issue 2 is grades and any other bad attitude (such as not doing chores, being rude to siblings, whatever else).
Issue 1: he's given you 2 ideas of what he wants - phone and dinner out. You cannot do the phone for financial reasons (not to mention he's too disrespectful to be trusted with a truly adult responsibility), but you could do the dinner even if it's an hour away, right? So do that. Don't mention the irresponsibility for now - just focus on the cost. Also, he likes Ghent stuff and while you don't understand it, it's his interest. Unless he's doing something harmful, cut him some slack. It's his birthday.
Issue 2: He's got a bad attitude and he's failing. So address his coursework (and anything that is distracting him from doing his homework) and any possible needs for tutoring. Maybe he's lazy and unfocused, maybe he's struggling legitimately with the content or study skills. So, work on empowering him to talk to his teachers and perhaps his guidance counselor to figure out what his problem is and how to address it. Offer to meet with the teacher(s) involved, with your son there. Whatever else he's not doing at home - chores or helping with other kids or having an attitude about you not cooking what he likes or whatever else he's pissy about - tie those things to increasing adult responsibility. He's going to want more freedom now, at 16, at 17. He's going to want a learner's permit. He's going to want a license and the chance to borrow the car.
So have a solid sit-down with him about what it means to be considered responsible enough to drive a $15,000 (or much more) death machine. Go over the expenses: insurance, gas, maintenance, excise taxes, etc. Get the info from your insurance agent or company about how much the insurance will go up when a teen driver is behind the wheel. It's math. Explain it as an adult having to learn to manage a budget. You can also show basic expenses for having kids - food, medical, school fees, clothes, utilities. You don't have to tell him what the family income or mortgage amounts are, but do the basics that he will have to cover when he is a newly-liberated adult. And someone who is disrespectful to his mother (or anyone else) cannot be trusted to be respectful to a police officer who pulls him over, or to another driver who cuts him off (hell, people have guns and shoot the person who was driving too slowly or who stopped for a pedestrian!). Someone who cannot remember to do his homework or turn it in cannot remember to use a turn signal or remember the rules of merging while coming off an entrance ramp into the flow of traffic. Someone who thinks he's entitled to an iPhone without working for it is not going to be responsible with an expensive car, especially if he thinks his parents are made of money. So tie his future freedom and privileges to his ability to demonstrate the skills and maturity that will be required of him, not just by you, but by the insurance company, the driver ed school, and law enforcement. It's his choice whether to play by the rules of life, you know. It's not you being a horrible, oppressive parent - it's the way of the world, and if he wants the perks of being a part of that, he has to play by the rules.
But I would deal with Issue 2 after the birthday. He gets a birthday gift and a dinner regardless of his grades.
