I would also recommend looking at AOPS. They have online prealgebra 1 & 2 classes starting May/June. Check the topics and see if they will fill the gaps your DS might have. My DS is taking prealgebra 1 right now and it is a well-taught class with assigned homework and enrichment problems (alcumus) offered as well as access to teaching videos on various topics. AOPS offers a good balance of instruction and problem sets that push the in class concepts further.