I found a few decent ones for larger kitties.
http://www.nextag.com/cat-harness/compare-html
And for kitties who don't like harness straps:
http://www.hdw-inc.com/walkingjackets.htmThis one has a size for small dogs too, so it's sure have a size that'll fit a big kitty:
http://www.kittyholster.com/index.htmlYou might want to prepare a carrier and make it a safe place for him. I use two that my kitties sleep in every day (unless I'm sleeping too...then it's Pile on Lyca time). If they get scared, the carriers are the first place they go, so it's handy during emergencies. Sometimes they just need an enclosed, quiet space all their own.
You might want to introduce your kitties on neutral ground, or get them used to each other's scents first before putting them together. Many can be territorial and will want to protect their home.