Is it just the hip popping or does he walk weirdly and/or show signs of pain?
If it is just the hip popping I would wait and see. I heard sometimes dogs get that in their growing phases.
If he shows discomfort, I would maybe go to an experienced vet, maybe X-Ray.
I doubt it's HD at this age, but you never know. Sometimes, I think we get too concerned and start to look for every single little possible symptom and we overlook that other dogs just behave the exact same way. I am too, guilty of this.
Frog sitting is normal at this age. I lured my dog into the right position when teaching sit.