It is on the tall side, certainly. An adult male boxer, per the US standard, is supposed to be between 22-25 inches tall. Bearing in mind that he is still only a 9 month old puppy, your guy sounds as though he's likely to end at the top of that range or possibly exceed it (most dogs would still have a couple of inches still to grow about this age).
It isn't certain that he'll end oversized, of course. Some pups grow faster than others, but faster doesn't necessarily equate to growing more

So you're really going to have to wait and see. One thing is certain though : you cannot consider your dog to be
at all small or undersized. He is already in the mid-range of the correct size for his breed, and likely will grow further.