Of course, any God would believe in himself, but would he believe in someone greater than himself? Would he postulate an external cause for the realm in which he lives? You might expect that a hypothetical God would be all knowing, so he would know if there is another greater God, but he couldn't know if there is another realm that he doesn't and can't know about. It is similar to any of us who can't prove or disprove the possible existence of a higher spiritual realm.
Another possible objection is that this God may be eternal and have no need of an external cause. However, how would he know he is eternal? I have no recollection of a definite beginning to my existence, but that doesn't mean that I'm eternal. Furthermore, there would be no way for him to be certain that he wasn't created with just the false memories of an eternal existence. Even if he were eternal, this wouldn't preclude the possibility of another realm that is outside his laws of physics and concept of time that could contain a deity that is greater than him and may have had some influence on his existence.
So if a God believes in an even greater God, wouldn't that be the better choice for us mere humans to worship? If, on the other hand, this God points to the lack of evidence of an even greater deity, and is therefore a heavenly version of an atheist, isn't it possible that this God would have a greater respect for open minded, terrestrial atheists and agnostics? If there is a God, could it be that that a path of healthy skepticism and avoidance of blind faith would be the best way to gain his approval?