A few ideas:
put skip counting by 2's to music, do it beginning with 0, 2, 4, etc as well as 1, 3, 5, 7, etc. Chant it over and over.
Lay out a number line on paper and put a chocolate chip, penny, cheerio, or other manipulative above each number. Now tell him to pick up every other one starting with 0(doing evens), that he has to 'skip' the others. As he picks them up he is to say the number of the ones he's picking up. Put the manipulatives back. Repeat, repeat, repeat. Then show him how to do it from 1 for odd numbers.
If you have a small frog or bear counter or other similar toy have it 'hop' on the number line over every other number.
Use index cards and write numbers to 20 on them, lay out in a number line and have him physically step on them, skipping every other one, saying the ones he steps on. Or hop over the skipped number, etc.
Lots of repetition!
PS> How old is your son?