Weedy great dane pup?

In all breeds, particularly the large breeds, some lines mature earlier than others. I had a male who looked nothing between 15 months, and almost 2 years. But he suddenly bloomed, bodied up and was fine. At 7 months, most large breeds are about at their ultimate frame size, but it may well take much longer for them to body up, fill out. Provided he's otherwise healthy (check for worms etc.) just don't look, for now!! It's probably just in his genes. If you can see his parents, it might give you some clues as to what's going on.