A custodial account is an account opened for a minor child and naming an adult as the custodian of it. Assuming the parents were the custodians depositing money into the account for the son/child when he is older. He is just using the money set aside for him to use with his parent's guidance.
Since the account where the money came from to pay college expenses for your son was a custodial account and not a "tax-sheltered" education account opened at the institution labeled for the specific purpose of educational expenses, then it was your/his money and is not taxable.
