My3, does he need to lose the weight?
I'm with FO...it's hard for men to ignore a challenge. And you can always throw in a sexual favor or two to spice things up a little.
I once heard of a man so desperate to lose weight that he wrote a check for $20,000 to a neo-**** group (he was Jewish) and gave it to a friend with instructions to mail it if he didn't lose X amount of pounds by a certain date. It worked!
I do believe, though, that the only way you lose the weight and keep it off is if you do it for you!