My boyfriend needs assistance with contesting a social security debt that he believes he is not liable for. This is in TN, and where do we look?

My boyfriend's mother signed him up on disability when he was a minor due to a learning disability. He collected a disability check for about 20 years. He thought he was able to work a certain amount and still get his check, but social security decided that he was no longer disabled and in fact has charged him with interest the entire allotted amount from the time he started receiving benefits as a minor. He owes more than $40,000 and doesn't know where to turn.