Does he have access to an iPad? DS5 (K) has been using a combination of Xtra Math and Dreambox lately, depending on his mood. I would only suggest Xtra Math if he is getting pretty quick at adding, subtracting, and or multiplying (you can change the settings). It builds automaticity with simple math facts - no teaching. It is free.
Dreambox has a free trial, but then there is a small fee. It is adaptive, so it will adjust to a child's level. DS enjoys it, but your DC may outgrow it pretty quickly. DS zipped through K-most of 2nd pretty quickly, but has been using other things for math the last couple of weeks. I am waiting to see if he shows interest in this again soon, because it is no longer free! I would suggest an iPad for this, though, because it sounds like some little ones have trouble with the mouse and DS handles it easily on the iPad.
IXL is more drill based - I don't know that it gives any instruction. I don't think I would recommend it unless your DS likes to just "quiz himself." DD's school uses it to reinforce what is taught, so we don't pay for it.
EPGY is pretty dry and the technology is not terribly up-to-date, but very solid on the math. We have DD8 using this as enrichment at home when she can find time - I wouldn't suggest this until your DS is a bit older.
My DC have both enjoyed Dragon Box (we have both versions; algebra game), but I have no idea if they actually understood the math they were doing. It is fun, though!
HTH!
