The best defense is a good offense. Take his ass to small claims court. You don't owe him money, he owes you money. In Texas, verbal contracts are binding, and judges are given broad powers as far as interpreting what exactly was contracted in the verbal exchange. The notion that some douchebag can hide behind the absence of a contract is bullshit. Also, here in Texas, even if you fail to win over the details of your contractual agreement, you still have recourse under the Fair Labor laws. In case you are wondering, these are the laws that (for example) protect illegal immigrants from being screwed out of their wages. It's a crime to have someone work for you and then fail to pay them. So, at some point, this could turn into a criminal matter for Mr. Douchebag to defend against, and he sure as fuck isn't going to be willing to face that, because in that situation, "your" attorneys are the prosecutors, and he is required to pay for his own legal defense. I figure it would cost him at least $2,000 to pay for an attorney to keep his ass out of jail if you filed charges against him for "Theft of Services". But that's a less-likely option, but still available. In terms of the civil dispute, for about $75.00 you make your case in front of a "judge" (in my county/state it's a justice of the peace) that has broad powers in terms of admission of evidence. Think Judge Judy or any other popular "court" TV show.
Going to small claims court would be a wonderful learning experience. Chances are pretty good you'll be there again, so there's no point in delaying the inevitable. Giving in to this douchebag is an indelible mark "LOSER" on your internal permanent record, and you don't need that, particularly this early in the game.
I called him a douchebag btw, because a guy with "plenty of money" that first hires teenagers, then tries to screw them out of their money is the very definition of the word "douchebag". Whatever his gripes might be, even if they are legitimate, he accepted them in advance when he hired you, given your age and (I assume) lack of experience. The J.P. is going to see it this way too, so when he tries to go down his 15 point list, I'm sure the J.P. will cut his nuts off and feed them to him in front of the entire court.
The fact that he has "plenty of money" is a good thing, because after you get your judgement, either he pays or at some point Sheriff's Deputy's show up at his house and/or place of business and start seizing property until they approximate the value of the judgement, meaning they'll take Mr. Monied Douchebag's plasma TV off the wall, while his fat wife stands in the Dining Room wailing in horror and embarrassment. Bring a video camera, upload the video and set-up ad-sense, btw. Also bring a bucket of KFC. His wife will blow you for an extra-crispy thigh and a drumstick, and thus your revenge will be complete.
Oh, and you can get court costs and sometimes damages. Court costs as in filing fees, but also compensation for the time that it takes to prepare for and file a suit vs. Mr. Douchebag. Make certain you bill for this time at the same rate you would bill for doing your professional work, as the time you spent playing touch-pee-pee with Mr. Douchebag is time you can't spend earning your money. So, if you are billing at $40 an hour, that's the rate you sue for when trying to recover the value of the number of hours it takes to file. One way of describing this is "opportunity cost". Also include driving time & gas, time spent on the phone, time consulting with experts on the best course of action, etc... Estimate high and chances are pretty good you get most or all of it.
Other people have made effort at communicating this last point, and I'll try it in my own way. This is a cut-throat world, and predators take advantage of weakness, and the only thing they understand is fear. As Machiavelli said, fear isn't the best way to rule people, but it's the cheapest and most efficient. This is the perfect opportunity to let the entire world that you are no one to fuck with. Failing to do this is an open invitation to the next douchebag to try the same thing. In order to prevent that, you must punish this douchebag, and leave his bloodied corpse hanging from a tree just outside of town, as a public statement to anyone else that might think about screwing with you.
The alternative is to cave, and that sets a precedent for losing within yourself. The next time something like this happens, it will be easier for you to give-in, as you've already done it once before.