Number lines helped my DS with both basic math and the ruler issue.
Draw a simple number line, with arrows at both ends, and label each tic on the line from 0 - 20. In bold color at the top, draw an arrow pointing to the right and label it "addition" (or plus or however your son refers to the function). In another color at the bottom, draw an arrow pointing to the left and label it "subtraction" (minus, etc.). Give him simple problems and demonstrate how the line works: 2 + 5. Start with your pencil at the number two, count over five to the right, and that gives you your answer.
Once he gets the hang of this, you can switch to the ruler and do the same thing (after all, a ruler is just another number line).
Also, find some worksheets that allow him to draw pictorial representations of the problem next to the numbers. Voyager Math for first and second is a good resource. Many of the worksheets (if you like worksheets) have boxes above, below or next to the problems so that the student can doodle a picture of the "number sentence."
For example: 4 + 5 = __________
Draw a picture representing the number sentence above:
* * * * + * * * * * = * * * * * * * * *
Now write the number sentence:
4 + 5 = 9
Eventually, they get the association.
Good luck!