How can I get a temp tag without having the title for the vehicle in my name?

I paid cash for the car from a private seller. He had just moved into his new house and said he misplaced the title but was going to look for it. It has been a week since I bought the car and found out yesterday that he is technically not the owner of the vehicle since he never had the title fully put in his name. I have insurance on the car in my name - which is worthless with no license plate - I just can't have it registered without the title. I live near the Baton Rouge, Louisiana area.