What needs to be done if a parent can claim a child as a dependent but the child already filed a return and said no one could claim him? Parent has not filed, yet.

After entering information for a dependent, (DOB, relationship, citizenship, marital status, income and paid more than half-support), TurboTax says I can claim him. The problem is he already filed for a refund and reported no one would claim him, (because he thought no one could). What needs to be done to correct this? Can I go ahead and claim him? Will he have to refile? Will he lose any of his refund?