What do I need to prove I deserve majority custody of my child

My wife and I, currently going through divorce have been separated for 6 years. We have a seven year old together. He was living with me last year. He went to his mother's the weekend before we were going to register him in school and then she went and registered him where she lived and refused to return him. My son was extremely upset. He said he hated his mom, and hated living there and wanted to come home. She finally let him come over for the weekend and my son told me he was being teased and bullied at school. His mother was leaving him alone at the apartment from 3 to 6:30. Her boyfriend was the handyman for the property so he would stop by once or twice in the three and a half hours to check on him. Then I saw his report card in his bag, he had all one's and twos. A student should be at the and four if keeping up with his grade level. He was far behind in reading and math. So I took him back and put him in.scjool with me. I will not allow her to see him. Am I in the wrong?