Where to start my 13 yo with writing?

We are new to the CM methods, but have jumped in wholeheartedly. I understand how and why it works and am working to apply it in all areas. I just feel like my 13yo son is sooooo far behind in writing. A little background… He has Tourettes and dysgraphia, so we let writing go a LOT in the past. So I just started having him do some copy work 2 days a week and dictation 2 days a week. I can’t dream of asking for written narration, I know, but he does quite well with oral narration.  His vocabulary and spelling is/ always has been far above grade level, but after about 2 sentences of writing, he gets hand cramps.  And his letter formation seems like it still needs work.. For example, all of his letters are the same size. Capital and lower case. Even tall lower case letters like l, b, d, k aren’t tall like they should be. Lower n and h look the same etc.   He understands this, but I guess it is just bad habits. He told me today that the dictation is just too much for him to think about while trying to correct his handwriting. He thinks doing the copy work will benefit him better for now. Does anyone have suggestions on how to bring him up to speed and correct bad habits without insulting his intelligence and having him write individual letters… He already told me this will drive him crazy. In the past, he has done GettyDubay italics books and will copy beautifully for months,but it never transfers to his regular writing. Am I to expect that consistent copy work for a long period of time will correct this or can someone suggest something else that might help?  I feel like we are at a disadvantage because he has dysgraphia and has used bad habits for so long. Also, how often each week is reasonable for writing?