I mean, the dictionary definition of 'selfish' says, "concerned chiefly with one's own personal profit or pleasure." so objectively speaking, he is being selfish. Whether you think he is doing the right thing or not is a matter of opinion, but leaving your HS team just before the season starts to do that is selfish.
Also, Southlake Carroll is in one of the wealthiest school districts in TX. That's where a ton of former Dallas Cowboys and other very rich people live. His parents aren't hurting at all for money, I can say that without knowing them lol. If Ewers wants to make that money and leave school early, that's his prerogative. But it's definitely not bc his parents need the money.