Good grief.
The boys were in the wrong but the man even more so. Some of you have called him an "elderly gentleman" but his behaviour, punching a child in the ribs and arm locking him is not gentle or gentlemanly.
The boys were showing antisocial behaviour which is not ok. They are also too young to be traveling on trains unaccompanied by an adult. Have the parents raised them with due care? It doesn't look like it. My kids are that age and they dont go out on trains alone and argue with adults!
In a ideal society everyone would behave respectfully but that's never going to happen. Personally I would have ignored the feet on the seats, but I would have intervened if a person was getting hurt. (By phoning the police or getting help usually)