Happy ignores his father Willy at this point because he feels rather embarrased by him. Happy is wanting to show off in front of the two girls that are with him and Biff. He's already told Miss Forsythe a string of lies in order to impress her and now he doesn't want the rather old and miserable-looking Willy around. But his wanting to show off is perhaps not the only reason. He probably also wants to distance himself from the difficult situation that has just developed between Willy and Biff. Happy often finds himself caught up in their squabbling and it's not surprising if he wants to escape the latest conflict.

Although he has his reasons, Happy's denial that Willy is his father still appears a low thing to do. However we have seen throughout the play that Willy has always taken more interest in Biff, even in their fights; he seems to have less time for Happy. This must surely have led to resentment on Happy's part and so he probably doesn't find it too difficult to deny that Willy is his dad. It means that there is no real closeness between him and Willy.