Signing as a cosigner obligates you to pay on the contract as if you were the only signer. What makes it worse, is you get all the liability (the payments) without the benefit (the car.) This agreement is a contract between you and the lender and absent some type of fraud or duress, I don't see how you can have the agreement voided. Consider having the agreement reviewed by a local lawyer to see if there are any defenses. If you alone are sued on the debt, consider suing your friend and bring him into the lawsuit on an indemnification claim.
Yes, you can sue him to have him indemnify you. If you are both sued and at some point in the litigation you come up with enough money to pay off the debt, you should consider buying the debt from the lender or having the loan assigned to you so that you can continue the litigation against the other borrower. If you just pay off the debt you will have to file suit against the other borrower so you're better off buying the judgment. You should have an attorney help you with this process.
You are responsible for making the payments to the lender. If the lender sues you, you can file a lawsuit (called a cross-complaint) against the borrower. However, you are still responsible for paying the debt. You could also file a lawsuit against the borrower for the payments you have made so far. However, it sounds like the borrower has no money, in which case you could receive a judgment against him but not be able to collect it. You may wish to hire an investigator to find the borrower and see what the situation is with the borrower. Good luck.