Let me be the fifteenth person to say TOTALLY NORMAL. Trust me, if your adults wanted to kill him they could, easily. They're holding back and teaching him the way he needs to be taught. Just wait until one of them actually hits him - he's going to go SCREAMING into the corner and stand there ki-yi-yi-ing for five minutes and you'll think they've disemboweled him. I've seen an older b.itch hit a puppy so hard his nose bled. As long as the adults are doing what yours are doing - swinging around with an open mouth and hitting him with the sides of their heads, or doing a big showy snap and roar in his face - you have absolutely nothing to worry about. He's got to learn to act submissive around them - do you see how showy and "big" he's acting, jumping toward them with his head and tail up? They're doing the right thing, telling him that's unacceptable.
How's his hearing? A boxer x with that little white on his head always concerns me, and any impairment is going to make him less able to hear their warnings and he'll get spanked more.