I find it curious that your son learned multiplication before addition, since multiplication is just a generalization of addition, so there may be a lack of understanding. I'm also considered a 'high functioning autistic' so I would be glad to help as much as I can.
Here gives the list of properties in the real number system. Properties of Addition
Personally I found visual cues rather than english clues a better indicator, but I'm not closed to the idea of other methods.
ex. Suppose you wanted to explain commutivity. ex. 3 + 5 = 5 + 3. have your 3rd grader draw three circles, then five. then count how many there are. do the same but have him draw five circles first and then three and count how many there are. notice they are the same number / 'picture'