Now I really need to take my name off the loan. What should I do if it seems he doesn’t want to cooperate? (i.e. not go to the bank to have the loan refinanced; not answer my phone calls; not reply my email). What can I really do in this situation?
Please give me some advice. Thank you very much.

Go to the bank yourself and ask them what can be done. State laws vary a bit on the terms of loans, but you want your name off the registration and the loan. The bank will know the laws of this matter.
If your ex is being that big of a jerk, ask the bank to take the car.
I would go to your local court house to see if you can gain ownership of the vehicle…
but beyond that… it is your debt now. Since he doesn’t want to pay it, you’ll have to. More than likely no matter what. The best thing you can do is somehow get ownership. I’m not sure how you’d exactly do that since different states have their own laws regarding this sort of thing.
You have my sympathies. I know your heart was in the right place. I’m sorry this little screw ball couldn’t man up and be responsible.
You need a novation from the bank…which they probably will not do unless your ex has improved his credit since you co-signed.
Contact the bank and ask about the possibility of a novation, talk directly to the loan officer servicing the loan and if that person denies you the novation ask how to appeal the decision.
Not much you can do. You made a commitment and now you have to honor it.