I almost hate to mention this at all-- but if the "homework" in the math classes he's taken is all answer and no 'work' then this may well be a matter of hearing mixed messages on the relative importance of doing this in the first place.
I saw this with my DD. Oh, sure-- Mom and Dad kept telling her to "show her work" but she did "fine" in math without doing that-- because everything was about entering the RESULTS (not the "work") into the computer, which then presented an output related to the correctness of said result.
Needless to say, this was not exactly helpful in convincing our DD to write things out. At all.
Just for her, I started doing spot-checks of her notebooks-- and if she got less than 80% accuracy, I specifically asked to see work related to that task, and if she couldn't produce it, she did it again.
I have no idea what I would ask for here, in an IEP, but clearly with kids who have difficulty, a lot of scaffolding may be needed to convince them to do it. DD is better about this now-- but far from what I'd call awesome. It burned her in calc and chemistry during her first few midterms in college, by the way.
And I rubbed her nose in it. Yes, I did. Because I've been telling her for years.
With that said-- does he understand WHY he must write out his work? That is, does he understand that it's not for his own benefit, but that of a third party to follow what his brain is doing? That really did help my DD to understand WHY she was doing it, and helped her to differentiate what needed to be on the page. Well, it helped, anyway. It didn't do so all at once, by any means.
Another idea would be to perhaps have a teacher select a representative problem and have him work through it with verbal explanations (on a whiteboard or smartboard), and SCRIBE the problem as he goes-- then coach him using that "transcript" so that he can see which steps probably should be written in order for another person to follow his logic.
