You have to take into consideration that rabbits are a prey animal - being restrained in a harness is mentally uncomfortable for them, and probably physically. I've only ever walked my old rabbit on a harness as he was tolerant of being handled by me, having it around him loose at first to get used to it and then with the lead attached. One time I had him out with my friends in the front garden, and he was spooked by something and ran, breaking his harness (which was specifically made for rabbits, and Clymo was only a small ND cross) and darting down a nearby ditch. Thankfully he got stuck in some dried grass and a friend of my brother's grabbed him for me - past this ditch is miles and miles and miles of fields, so I'd have never seen him again if it wasn't for my friend's help and the dried grass blocking his way. That was the last time he wore it, and I only ever put a lead/harness on him again when he was much older and not as active. We sat by that ditch after having a walk around the block, and he came up to me after a few minutes and started nuzzling and scratching at my arm as if to say 'take me back home!' lol!
My advice is simply do not stress him out. He doesn't need to be on a lead/harness, he doesn't need to be walked, and it could end in heartbreak for you if you lose him. If you have a garden, buy or make him a lovely large run, or just let him have a safe place to run around inside. It's really not worth the risk.
