Hi Shane
Glad to hear your pup is doing well. i've only had one other puppy, a cocker spaniel, and he seemed to go through growth spurts too, but perhaps not as dramatically as my boxers. (perhaps since boxers are bigger, you notice changes more?) Anywho, i wouldn't be too worried about his growth spurts, he's only a little over 8mos, and he's still got lots of growing to do. He will probably finish growing in height by 1yr (and you can feel for his "growth plates".. like where his shins would be.. if they are still bumpy than he's still growing taller, if they are smooth, he's just about as tall as he will get), at that point, he will start filling out. Many male boxers don't fully *look* mature til they are 3, but usually between 2-3 is what they will look like fully grown and filled out. Hope that helps!
-Kat
ps. aren't Boxers the greatest! they've got the most personality of any dogs i've EVER met. its soo easy to fall head over heals for this breed. :D