According to all the professional advice I've seen, 8 months is a little young to change onto adult food, so if I were you I'd buy another bag of puppy food. Then, at about 11 months, buy a bag of adult food and start mixing the two, gradually increasing the proportion of adult food in the mix until he's exclusively on that. If you just suddenly switch him over to the new food, he may well refuse it.
As for the puppy Vs adult food debate, my own opinion is that an adult food (as opposed to an all-stage one) doesn't contain the right mix of nutrients for a puppy. It should be obvious that a puppy, who is growing at a much faster rate than an adult dog, has completely different dietary needs to a mature dog who has reached its adult size. You wouldn't feed a human baby an adult diet, why would a dog be any different?