I wouldn't
expect to see well-defined muscles until he's about 2 years old. Of course it is possible that he may fill out sooner than that (or even later), but 2 years is generally when you can expect to see the adult physique. Boxers younger than 2 years are still considered to be puppies, and most are pretty lanky up until they reach maturity. My female, Caney, first started to fill out around age 1.5; she's now 2.5 yrs and still I notice little things about her physique that are still changing.
From your description, Cyrus sounds to be right on track. And he's 70 lbs already??

BOY, is he gonna be BIG!
Just out of curiosity, how much are you feeding him and how much/what sort of exercise is he doing?
__________________
Zoë
Creek - Fawn Female, born 03/27/2001
Caney - White Female, born 03/26/2007
Photo Album