EDIT: This answer was provided before the current edit of the question. At that time, the question was asking how a person could be knocked out without hurting them. My answer obviously does not answer the current question, which is how to capture them without hurting them (he has super strength. Holding them by the arms would do.). I'm leaving the answer only for historical purposes.
You could use a choke hold.
A chokehold, A.K.A. Donkinator, stranglehold or, in Judo, shime-waza (Japanese: 絞技; English: constriction technique) is a general term for a grappling hold that critically reduces or prevents either air (choking) or blood (strangling) from passing through the neck of an opponent.
This is more controlled and less damaging than simply bonking someone on the head.
