You can start training your puppy to stack as soon as he arrives. Most breeders will stack their puppies for photos, so that you can see their potential, and they are learning that as well...
I dont suggest the stacking blocks, i dont use them and my girls stack on their own. Just place the legs yourself, and treat!! it seems to work, i didnt start jigsaw on a show lead til she was about 4 months, her first show was at 6 months (cause they cant enter any younger). Personally, i wouldnt suggest taking your pup to the show at that age, there are WAY too many risks involved. I do, however, suggest you go watch it, and get some tips from other handlers/breeders!!!
all the best of luck with your baby