At that age, I usually find a cheap one at a Petsmart or Petco and start having them wear it empty to get used to it. Maybe a couple water bottles if I want to add weight. Then I look into buying a permanent one at about 1.5 years. I'm really liking what I'm reading about the Mountainsmith. I think that's going to be Kaiju's adult backpack.
But until then, he has a super cheap off brand one that I picked up at Petsmart for like $10. So it doesn't matter if it gets beat up during puppy antics or he outgrows it.