My question is, what level of physical punishment could his father give him before it crosses from 'reasonable corporal punishment' (in the minds of the characters) to assault? What kind of blow would a teenager raised in a fairly strict, traditional Japanese household expect for repeatedly and openly defying his father?
Specifically, I want to know if an open-handed slap to the face would be reasonable? One hard enough to leave a mark several hours later?
Googled 'corporal punishment in Japan', mostly school focused results; looked on the Wiki page for corporal punishment, which says that it is still legal to punish one's own child under Japanese law but not to what extent.