The explanation at the KH tonight (Oral review) was that Saul knew who he was, he knew his name etc. but after he killed Goliath he asked again evidently wanting to know what man had fathered such a boy no doubt with the idea there may be other similar members of the family who would be useful to Saul's army.
Don't know where they get this from but that was the answer for what it's worth.
George