When you cosign, what you are saying is that if the primary signatory (your son in this case) does not pay, you will. If your son has stopped paying, the lender will expect you to live up to your promise. They lived up to their side. They made the loan as you requested, on the basis of your signature.
There are only three ways to get your name off this loan:
Get your son to refinance the loan under his name. As you note, it seems unlikely that he would be able to do this, even if he agreed. Perhaps his mother would be willing to cosign (although if she asked, we'd recommend that she not do that).
Pay off the loan (either you or your son could do this).
Declare bankruptcy and get the loan discharged.
Some people are recommending a fourth approach, which is to just ignore the loan. This won't get your name off the loan. After a long period of time (probably four years in Texas), the lender will stop trying to collect it from you. In the meantime, it will show as a bad loan on your credit report (and your son's). And they can harass you to collect the loan for that period of time. In addition, they may be able to sue you and compel you to pay the loan. Talk to a lawyer for more information on that.
Cosigning for a loan is a big deal. If the primary signatory does not pay, you are responsible for the loan. That is what cosigning is. You should not do it unless you are willing to gift the person with the money.
Are you willing to declare bankruptcy over this loan? If not, your only real options are to pay it or ignore it. If you ignore it, your credit will take a hit, probably a big one. Paying it would be expensive, but you can negotiate a fix for your credit as part of it. The absolute worse thing would be to switch between options. If you're going to ignore it, accept that it will ruin your credit for years. If you're going to pay it, negotiate that soon. As already suggested, you may be able to get the lender to repossess the car, which will reduce the amount owed.
Negotiating a credit fix may require a lawyer and an escrow company. You agree with the lender on an amount which you put in escrow. They get the money when your credit report is fixed.
If you plan to ignore it, you should also hire a lawyer unless you have no assets or income. Because you really need to be informed on what the lender's options are in terms of forcing you to pay.