I'd prefer they make Leeroy take one damage for each spell cast on him. The card itself is fine. It's when he gets stacked with multiple buffs he gets ridiculous. With one damage per spell he could only be buffed once with his two health. That seems reasonable.
Leeroy is reasonable already, because he has a downside big enough to make him almost unplayable on curve. Using him as anything but a finisher is a waste. He's so situational that you have to build the entire deck around him and plan the entire game to maximize his potential. That sounds to me like what a legendary card should be.