In the MAC, Adrenal Defense is listed as requiring 40% activity as its base.
However, Martial Arts Styles can incorporate Lesser or Greater Adrenal Defense into their styles. You can adapt these to be additional talents that he can take (i.e. they require the "Martial Arts Training" talent to be taken)
Lesser Adrenal Defense [10 points] - Character can ignore the item in hand limitations for weapons associated with his martial arts style (would suggest only 2 weapons), and AD requires only 30% activity, and it does not count against the 3 actions per round limit (but does count against the "no more than 100% activity limit").
Greater Adrenal Defense [20 points] - As Lesser Adrenal Defense, except may have 2 weapons, plus one weapon every 10 ranks that may be used as kata weapons, which ignore the "no large objects" rule. Also, only requires 10% activity to maintain each round.
By giving him these options, you are putting the choice back into his hands, and offering a way to allow what he wants without unbalancing things unduly.