John Wesley on 1 Samuel 17:55

Whose son - David had been some considerable time dismissed from Saul's court, and was returned home. And therefore it is not strange, if Saul for the present had forgot David. Besides the distemper of Saul's mind might make him forgetful; and that David might be now much changed, both in his countenance and in his habit. I cannot tell - Abner's employment was generally in the camp, when David was at the court; and when Abner was there, he took little notice of a person so much inferior to him as David was.