WIY: You’re thinking of Hashem as we believe it, in Judaism. For someone whoisn’t sure what to believe, why do they have to believe god is perfect? Maybe he’s just some super-powerful being who can get bored?
As for an obligation of hakaras hatov, yes, in an ideal world everyone would give hakaras hatov when it is due. Sadly, that is not the case. Besides, this guy doesn’t believe the powerful being gave him everything. The being created the world, and left it to run itself.