Maybe he's also over-thinking it? I mean, concepts are great, but the most important part, in terms of using it, is being able to use it.
I know when we first started calculus in Gr. 12, my classmates were all hung up on understanding it. I was never all that "good" at math, so I just approached it from a practical angle, and figured out what the kinds of problems were and how to solve them. I vaguely got the concept, but I didn't worry about it too much. And I did ok - this actually served me well even through my 2nd year university math courses (which I took because I was going to be a math and science teacher, and because I liked them, sort of.)
I also found that I liked long complicated problems better. It was like a form of meditation to dive into a deep problem and go through each step methodically. I'm no math whiz. It wasn't easy, but I like to believe that nothing worth doing is ever easy.
But a good tutor would help, as well as reviewing earlier stuff. Is it the Miquon program that has the placement tests? Whichever one that is would likely be a good place to start. Or how about Danica McKellar's book "Math Doesn't Suck
". We got that for DD when she was in Gr. 8, but she found it a little too basic for her. It's all girly, but it goes over all the pre-algebra skills a student might need in really friendly ways. There are also several books devoted to math fears and phobias that might be worth investigating. Also, if he's going with textbooks, that may be more complicated than necessary for getting himself up to speed - something like All the Math You'll Ever Need
. He'll need more than that, obviously, but it might help him get up to speed. A librarian might be a good person to ask, as to what kinds of books might be helpful. Or maybe the videos from Khan Academy
Good luck to him!