When he falls asleep the powers take over, Fight Club style. Or he could experience periodic blackouts and bouts of amnesia which are a result of his memory blocking out the usage of his powers. Or you could make him have anterograde amnesia, which doesn’t allow him to form new memories past a certain point (that would make the character a little hard to deal with though) You could also do something like Hulking out but he just has no memory of it when he wakes up.