He's not focusing on getting his work done, or in on time. I'm wearing myself out prodding, coaxing, encouraging. He wants to go to college to be a mechanic. I'd like to help him get motivated, but, he's easily distracted. He's a good kid, but, he's wasting time. He lost 2 take-home tests this weekend. His Algebra II teacher said if he completed & passed them he'd pass her class. He's dilly-dallying in his English classes too. He's 18, so I cannot legally enforce any restrictions on him. He's either texting, talking on the phone, online, playing his guitar & drums, or watching TV. I'm %100 behind him, how do I get him to get in the game?

Take away the computer, the video games, the cell phone, all the music to play or listen to, the TV, and the tablet/iPad. Everything he does has to be done (neatly!) with pen/pencil and paper (*cough* the way I used to do it in high school). Make sure his teachers know this.

Make an appointment with each of his teacher and attend with your son. Listen to their expectations and what will happen should he continue on this path toward failing his senior year. As Wondergirl said, remove all fun from his life and do not allow him to have these things back until his work is turned in on a regular basis.

The problem is you, your attitude, you stated you can not "legally" enforce, BULL
He is living in your home, you are paying his bills you can enforce any rules you want.

Take the computer away, take the phone away ( who pays for them anyway?? ) ground him and start being a parent, It is easy to see from the post why he is the way he is, You have allowed it