Usually the time to sue is when you were sued. You could have filed a cross-claim against your former boyfriend at that time. Whether it was worth the time and effort, and the money, is another question. Is the former boyfriend financially solvent? If so, and if the statutes in your state allow you to sue, it may be worth it depending on the amount of money at stake. If not, you are probably out of luck.
As to the witness, the former boyfriend cannot be made responsible to the credit card issuer for the debt. You signed up for the credit card. He did not. You can't make him responsible to the credit card issuer, although he may well be financially responsible to you.
I am not an attorney in Ohio and do not know Ohio's laws. The best time to have dealt with this would have been in 2008 by filing a cross claim against your boyfriend. Now, it is possible that you have a statute of limitations issue. You need to ascertain that quickly.
The other concerns are more practical. Is it worth suing the boyfriend? Does he have the money to repay you? Will you be able to collect from him if you are successful?