I can't say if that's normal or not and I don't believe i have any goslings that young anymore to compare him to. I certainly wouldn't fret over it and would say that as long as he's healthy and doing fine it's nothing to worry about. Seeing that you only have two and assuming they're have plenty of feed and grass it would not surprise me that they're growing quickly.
At one time it was prefered even for the larger show and exhibition type birds to be rather paunch less and have a smoother underline (though still show the desired double lobes). It was thought at the time that such birds with large lobes were the result of crosses with toulouse. It would appear though that being a heavier breed the large paunch was not a hinderance to show birds even and many today have ample paunches (though as the '01 Standard states "stern round, full, free from bagginess") and what that looks like is open to every judges interpretation. (Keeping that in mind the '01 edition makes no mention of lobes at all, but as shown by today's breeders and judges dual lobes are what is desired in this breed.) I realize that with your birds being of generic Africans from Metzer's this info may not apply to your bird in particular, but it does dispel a few of the other questions on the thread and with that being said we don't know what is exactly in his background and Metzers may have large birds in their breeding pen or the combination of genes was just right to lead to early develpment.