Mine two little ones also use Buddy Belts. If there's some resistance, start slow and reintroduce the harness as a 'fun activity'.
* Have him sniff the harness and celebrate "gooood boy!!" and give him a treat.
* Get him to put one foot into the harness and then quickly take it off again, repeat the party and treats.
* Get him to put two feet into harness and quickly take off again, party and treats.
* Have him wear it for a few minutes, party and treats.
You get the point 
Basically, do this multiple times a day over a span of a week or so, in gradual steps, starting by just sniffing the harness and working up to him wearing it all day at home and then wearing it outside.
With mine, they love the harness on because they know it's time to go out (they didn't always
). But each time I take it off after our walks, I give them a treat, because then they sit still until I'm ready to take them to the tub for cleaning.
