Billy got granted a super-powered female alter ego, while his sister Mary got a male one (you may need to have read/watched Shazam for it to make sense)
I’m only vaguely familiar with Shazam aka Captain Marvel. Essentially, Billy and his foster siblings get powers from a wizard. This wizard gives them powers of these old gods/deities. Conveniently the sex of those deities match the sex of the power bearers. I’m guessing the powers in this comic don’t see sex, so they gave Billy the female deities power and Mary the male deities power. I don’t know what “boy ong” means though
In the canon, I think they just end up looking like the grown up version of themselves. My reasons are because in the movie, Mary (the oldest) is played by the same actress when she transforms and in the comics (Kingdom Come, I think) years in the future one of the Justice League members sees a person who he thinks is Captain Marvel, but it's just grown up Billy Batson.
Better yet, Luthor has Batson in his entourage, so everyone thinks Captain Marvel is rolling at his side all the time. Until Batman realizes it's just... Billy Batson.
Unless there's some other meaning for it, ong is some kind of slang, it's short for "on god". I think it's used in places where one might otherwise say "I swear to god" instead. Of course like that other person said it could also be a weird typo, but it seems deliberate to me.