I think it sounds like David was in training to be Hooper's successor by Season 14.But, to be fair, David ran the store many times on his own. So, even though the "nose" episode aired late in the season, it wasn't unusual to see him alone behind the counter
He was an old man.Based on what was said about episode 227, it sounds like Mr. Hooper was experiencing health issues in the early stages even during the second season.