Lynne, i'm not a lawyer, but i just wanted to tell you that YES, nothing will prevent him from marrying in Turkey as your marriage was not registered there and therefore they have no record of it.

My Turkish husband also left me & our kids shortly after he got his US citizenship, went back to Turkey and remarried there without ever filing for divorce in either the US or Turkey. I have since registered the marriage in Turkey via the Turkish embassy in the US and his Turkish marriage has been invalidated.

Good luck.