WOW he is a houdini
Perhaps trying a different style or brand of harness? I tried about 100 different types until I could find one that I felt safe having Simon in (he's a puller) and now use Cindy's harnesses (chattiesmom) and I really like them.
Don't take this the wrong way, I'm only asking because I did it but are you sure you are putting it on the right way? I struggled with a step in for days before I realized I had it on ALL wrong. Then when I finally figured it out I found that it loosened up while we were walking.
Good Luck~