
Good for you not piling in at Championship show level with a puppy!! So many people don't start low these days, and often get put off when they don't do what they hope for. Years ago, nobody bothered to enter a Champ.show until/unless they started doing some significant winning at Open shows ...... yes, I know show entries are high, so why bother with the Open shows, but as said, in order to avoid disappointment, especially for the novice exhibitor, it's often far better to support the Open shows to cut your teeth on.
Whether you go for Junior and not puppy AVNSC classes would depend on whether numerically your breed is high, so you might find yourself up against older youngsters with your baby. Whether you do the AV Puppy class would be up to you although I'd not enter in too many classes with a puppy who is new to the ring. If there are classes scheduled for the breed at the show, you'd not be able to enter AV-NSC (not separately classified) in any case.
On balance, to make these first outings enjoyable for both of you, I'd go locally and put him in AVNSC Puppy, assuming there is one, and see what happens. At this point it's all about experience after all!! I ssume you've had him to Ringcraft?
I'd not enter in loads of classes with a puppy - again you don't want to put him off showing!!
ps If he won Junior (with no breed Puppy classes on offer) and was the only puppy in that class he would be an unbeaten puppy. So could go in, later on if you wanted to hang around that long, for Best Puppy in Group.