I would report him for this, but be aware this is not uncommon behavior for boys this age. They are walking hormones.Not saying its ok, but its not some rare thing either.
I recall a couple boys getting in trouble for playing "tune in tokoyo" with the girls the grade above us. Just total buttheads. That was many years ago.
In many ways the sexuality young people exhibit has changed, but smacking butts and grabbing boobs is not a new thing, sadly.