Praise your boxer puppy for rolling over each time he does so, and pat his belly, saying, "Good rollover! Good!" When he starts to link the words with the action, give him the "Roll over" command, and help him along by gently pushing him into the belly-up position and patting his belly.
Let go, praise the rollover, and give him a treat. Pretty soon, he'll link the command with the action and reward. Then you can embellish on the trick by gently moving his paws back and forth while he's on his back, saying, "Do your exercises!" Or hold him very still, and tell him to, "Play dead," praising him with the phrase "Good play dead!" and a treat.
Boxer - Dog Lover
