Please keep in mind this is just my personal opinion, I am not an ABKC judge or representative, but I'd like to think I'm at least decent at checking a dog's structure. =] I plan to show ABKC myself soon, and have a number of close friends that do show ABKC that I am learning from.
If you're showing ABKC you'll probably have the best luck showing him in the Classic Class. Are his eyes blue? It might just be the picture, but they look blue, and that's a fault in the show ring. As far as his rear goes, he may just be going through an uneven growth spurt. It'll also help to teach him how to stack properly, since he's not standing stacked in these pictures plus the angle it's hard to tell how his rear really looks. Back legs are cowhocked, but with proper stacking and training you can sometimes correct that. Pasterns look weak though, at least in these pictures. Tail might also be a bit long - still hard to tell with this angle. He also lacks significant cheek, but again, he might grow into that.
Overall, I think showing ABKC would be fun for the two of you, but I don't think he'd do that well in the ring. I don't think he's quite "Bully" enough for the ABKC and lacks breed type to win, and breed type is a super important thing in the ABKC.
But! I may be wrong, and if so I will happily eat my words! Either way, I wish you and Otto the best of luck. =]