Pulling out my dog trainer hat...
You should never give a command more than once. If you give it and they don't listen, you should - without saying a word - place them into the position. Down is a tough one...it's submissive...but they need to do it when told.
What worked for us...when Mabel went down on her own, she always got a cookie and TONS of praise. When I had to help her, she got nothing. I didn't yell at her or anything, I just didn't praise her or give her a cookie. I also tied her meals into it. When it was meal time I'd tell her "Down." If she went down, the food hit the floor and then I'd give her the "free" command, which means she can get up and eat. If she didn't go down, I helped her down but the food went on the counter. When I freed her there was nothing for her to eat. I then waited a few seconds and tried again. It really didn't take her that long to get it.
Once she had it consistently with a food reward, I slowly started weaning the cookies out. Sometimes she'd get one, sometimes she wouldn't. She never knew when she'd get it though, so she obeyed every time. Now she knows she won't always have a treat for listening but she'll always have my praise. Sometimes I swear she cares more about that anyway.
Anyway, consistency, patience, lots of praise. He'll get it!