first off, atk is a pointless stat for him. his damage scales off defense, so attack is quite literally useless.
whether to build him as a dps or a debuffer depends on what you need more right now. you might be able to do both in the long term, but it will be challenging at the moment. if you opt to go the dps route, you'll want to focus on crit rate, crit damage, and defense. if you'd prefer to use him as a cc champion, you'll be more concerned with accuracy, speed, and defense (you'll want speed for the dps route as well, but since youre building for more stats total it will likely end up being lower).
neither stun nor frost is a particularly good set for him. frost is used most often in pvp and works best on a provoker while stun is most effective on champions with all AoE attacks (and could end up being redundant on him, since his attacks can already stun and you cant stun someone twice). more importantly though, you're way to early in the game to be worrying about set bonuses. stats are far more important than sets and you should be focusing on making sure he has gear with good main and substats. if you have to use gear from 6 different sets to do that, that's fine.