Yes he's surprisingly hard to like at first, yet it quickly becomes clear why, one of the first things they show is Billy sticking up for and defending this new family, that he's outwardly suposedly rejecting, and doesn't seem to want to connect with.
Did you at least read that part?
When bullies pick on them, he can't help but defend them. Immediately shows you his true nature and instinct is defending the small/awkward/downtrodden, ...his new family.
He then uses himself as the decoy, so Freddy can get away.
Revealing who he is.
It was pretty
clear from the go, he's a tiny bit more complex, so as the story builds to a kid in conflict, they are showing you two sides to the character.
So it's not just about getting power because you are "born perfect", but the choices you make once given power. This is all paralleled wonderfully against Teth Adam's origin which parallels Billy's to an extreme, who also is chosen to get the same power, but makes a different choice.
It's all there.
When the Wizard explores his mind, you see exactly that: Two sides, one lashing out, rejecting, showing what he could be. The other reaching out to people (and animals) in need, helping them.
In it's simplest most whimsical form, any kid could grasp it.
Because of that, the big revel at the end, means all the more, it carries weight; why he was chosen, what he can do, what he chooses to do with the power means something!
Read it!