I think subtraction was hard for my emotional dd because she didn't like to take things away. But once she learned about borrowing, it became emotionally more satisfying to her. Sounds crazy I know.
I've found that DS also is just plain on a different 'developmental path.' Also if an idea get stuck in his head, it can be really difficult to find and build a road around - I'll use an example from my past.
In first grade my writing was really messy. My Math sheet were a particular problem. My mom sat me down with our next-door neighbor who had a backround in teaching. She asked me questions, and drew out that I believed that the larger digits should be written larger, (ex:8 should be twice as big as 4 because it was more grown up- obvious - right?)
She explained that I should make them all the same height. And I was willing.
so my handwriting was still messy, but at least the teacher could read my math sheets.
My idea is that you might have more luck with Division, because the emotional backing is 'sharing equally amoung friends.' Then you can come back for subtraction.
I also loved-loved-loved the number line that the teacher painted on the floor in the front of our 1st grade room, and loved to be chosen as the one who walked back and forth to demonstrate. If i'm doing a simple addition or subtraction problem with negative numbers, I'll draw a crude number line on the top of the page just to check myself.
Smiles,
Grinity