This is an interesting question, and the answer depends upon the perspective. Happy has been subjugated to the athletic Biff. Clearly both parents have focused attention on Biff to the exclusion of Happy. Happy's adult life reflects this need for attention in his womanizing ways and his lack of commitment to anyone or anything. He still seeks validation as evidenced by his constant assertions of losing weight and promising to get married. Thus, it is understandable that Happy has sought attention elsewhere and even made his presence scarce around his parents' home.
However, the word 'justified' is very strong, and the answer would have to be no. Happy is not justified in denying his father. First, the play gives no evidence that Willy denied Happy. In fact, Willy includes Happy in his declaration of "my boys" as he contemplates moving to Alaska with Ben and at the prospect of the Loman Brothers Sporting Goods store. Happy is never denied; he is just not the primary focus.
Furthermore, Willy is clearly sinking into dementia. He is slipping away from reality and making an unfortunate situation more pathetic by his oddly timed outbursts at dinner. Leaving him in this state to sit babbling on the bathroom floor cannot be justified, even if Happy felt slighted as a child.