Alex is way too timid to be in Slytherin. His dark turn is entirely based on him being hurt, but he probably doesn't have the personal ambition to keep it up.
Alex isn't adventurous, either, which would make it hard to see him in Gryffindor.
He's totally willing to work hard and tirelessly on something, but he's too invested in learning and expanding the possibilities of what can be done to really be as regular and faithful as a Hufflepuff.
Alex is definitely a Ravenclaw. Like his father and his guardian, Alex's chief pursuit is knowledge. He's creative and inventive and loves learning for learning's sake. His mind is his most defining trait.