I just received a letter that my 16 year old grandson is now in legal custody. His mother would not let me talk to him for the last two years.
Can I get legal custody since I live In Louisiana?

Sure take it to court. You are the closest relative. The state is not in the business of taking care of children. They want him to be with his family. I urge you to get him. She obviously was an unsafe mother and the husband seems to be out of the picture. You are his closest relative (next of kin).