DS isn't getting math

[AVDesigns]

DS is really behind in math. He has NO number sense. He just turned 7 and with his aspergers and developmental delays I expected some difficulties, but it's beyond what I expected.
Tonight he had subtraction (which is harder for him than addition) and I tried to get him to do 1-1 on several different problems and he kept saying 3. I even showed him 1 pencil and then took 1 pencil away to show him that leaves 0 and he just wasn't getting it.
I feel like such a failure because I can't figure out how to make him learn it. I don't want him to struggle.