At 8 months of age, he is almost certainly still growing vertically - though not as quickly as he would have as a younger puppy. At a rough guess, I'd say he'll probably grow another couple of inches.
It's hard to recall exactly, but I think my dog was about that height - possibly 22" - at that age. He ended up right in the middle of the standard, 24" tall and weighing around 31-32kg (though technically he's a bit overweight at 32kg - 30-31 is more ideal). 32kg is about 70lb, btw.
As for the growth plates, those are the soft areas of bone that exist at the ends of all the long bones. The most obvious, I think, are the ones at the dog's wrists - they're big and bulbous on young puppies, gradually diminishing until the adult dog (with closed/hardened plates) has a pretty smooth outline on the legs. Here are a few photos from Henri's youth where the growth plates might be obvious enough to see the changes:
5 months - quite large lumps by the pasterns: Boxer Dog Photo Gallery & Postcard - Henri, cutie
11 months - reasonably smooth pasterns: Boxer Dog Photo Gallery & Postcard - Happy dogs
Adult - no sign of growth plates: Boxer Dog Photo Gallery & Postcard - Henri - squirrel up a tree
Boxer Dog Photo Gallery & Postcard - Impatient
In both adult pictures, incidentally, he is about 30kg - perfect weight for his size
It's more toward middle age that he crept up in weight, and I have to monitor his food carefully now to keep him below 32kg. For a dog of his size/build, 70lb is overweight.
In case useful, here's a previous thread on the same topic. The link to the skeletal drawing might be useful to help locate the pastern joint where the growth plate is evident Question about Determining Age and other questions (long sorry)