There was probabşy other claimants. By claiming to be her son and getting away with it he probably stopped a civil war that might start compared to if he did not and there was half a dozen other people with same claims. It would be a show of force about his support so to speak that would invalidate every other claim.
Except you know he would have to claim to be 110 per what the GM said, it is not a tenable lie on the face of it.... unless of course it is true.