Thanks everyone for the feedback. It definitely gives me more options for DS. He does not complain about IXL but he is unsupervised and enjoys the freedom of picking any problems he wants to work on. He does not seem to care about wrong answers but loves the challenge so of course goes straight to the calculus problems. I am hoping that he gets exposure to more of a variety of math subjects so he does not have learning gaps. He tried alcumus last night and really enjoyed the number theory problems. I am thinking about getting him the intro book as a start.