Eligibility for Social Security Survivor Benefits?

My boyfriend's mother passed away when he was 15 and he is now 22. He was receiving Social Security checks that were addressed to him from the time he was 15-18. He moved out of his father's home (who was not married to his mother at the time of her death) last year, and he is now refusing Josh (my boyfriend) the remaining money from his Social Security checks claiming that they are his even though the checks were made out to Josh.

We were told that after the child of the deceased turned 18, the surviving parent is required to return any leftover Social Security money to the Social Security department and then the Social Security office writes Josh a check for that amount, but that never happened. Who is entitled to the money after he turned 18 and how can he prove it if it's his? (Also, his brother who is 13 years younger than him is currently still receiving benefits of the same kind, so that's why we are so sure that the money is Josh's. His grandparents are raising his brother and have assured Josh that the money is going to what his brother needs and when he is 18 that he will be able to keep whatever is leftover.)