Silia, isn't it reasonably well established that you have a tendency to panic?
As such, you drop the announcement on J that you have informed voodoo zen poly master C that you're hot for him, and sensei guru poly man says he's hot for you, too. (Your earlier posts and this one made C seem like a dreamboat of sorts.) And J had a little more trouble dealing with this than expected?
Is this really a surprise?
And so you sensed his trepidation and immediately began backpedaling and offering up all sort of assurances that C would be contained or disposed of?
It sounds like you laid this thing out to J and then tried to take it all back when he didn't fart rainbows and glitter with excitement. Poly life is a change from mono life. There is a lot of mono programming that has to be disconnected and unlearned before someone really transitions into poly life. Even then there's all sorts of realities of poly life that take effort and practice.
Step 1: Realize that you're poly
Step 2: Freak out a little
Step 3: Stumble into a poly thing
Step 4: Run away?
Step 4: Give it a little while to let J adjust. You're not the only one that's having to change.
Poly adjustment syndrome affects everyone a little differently, but until you actually face real situations, it's hard to tell how someone will be affected.
So let's imagine I'm J...
If I were J--a seemingly nice, mono dude--and my partner told me that she wanted to love others, that would need some adjustment. If the first person my partner brought home as a new partner interest was a seasoned poly veteran who happened to also be the person to introduced my partner to poly life and happens to be in a seemingly successful long-term poly relationship, I might shit my mono-britches. Virtual J is wondering if swami saint poly priest C will add his beloved to his ever-growing stable of poly princesses, and once the indoctrination is complete, maybe beloved won't want old J anymore.
Can you see how this might cause a little freak out? The concept that a poly will not run away and abandon a mono can be a really hard notion for monos to accept (MonoVCHPG-RQWHGJNetc. chime in if you want).
Rather than call his freak out and raise him a emo breakdown, reassure him of his wonderfulness and then give it time. A lot of poly life is improved by giving things time to properly simmer. You need a night out and a nice dress to bring yourself to a boil. Maybe J could use a day or three to cook off some excess worry.
Keep the communication lines open, but don't tie them up with a lot of, "ok, are you ok now, ok?"