My son did IXL initially. It was great in giving him loads of practice problems but he would have to miss the problems he did not know and then read the solution to learn. Not the best approach.
He then did EPGY for several years and liked it. They taught topics and then had questions. Once it got to geometry though we saw many flaws in their program.
We then learned of AoPS which has been great. I wish we had known about it first. They have Beast Academy which I think are textbooks, so not online, but I would think if it is from AoPS it should be excellent.
If I were you I would use the Beast Academy program until he is ready for the AoPS courses.
