Step 1: When he lays down, click and then treat
Step 2: Repeat this several times
Step 3: Say "Down!" when s/he lays down and then click and treat
Step 4: Repeat this several times
Step 5: See if s/he understands. Get them to stand up and then say "Down!" If s/he lays down, click and give him a reward. If s/he does not, go back to step 4.
GOOD LUCK :)