OK, we need to get back to basics here.
What he needs is lots and lots of crumbles or pellets. That needs to be 95% of his diet. He's a chicken. Rat food is not appropriate. Grapes and stuff are great for a healthy bird. He's not healthy. He's emaciated and that much emaciation is one step from death, so he really needs his crumbles or pellets. Not to scare you, but this is indeed very serious as you can see and have indicated to us.
He's too weak to perch incidentally. That's why he fell when he jumped. He has no fuel.
For now, please don't give him grains. The reason is that his digestive tract is so stressed that he may not pass them, and you don't want to risk slow crop. He needs food that is just easily dissolved. (Again - crumbles, pellets). If you want to spoil him, he can definitely have boiled (not scrambled) egg yolks. I bet he would LOVE that and it's a very good thing to give a bird to build their blood back up.
Other than that, I'd give vitamins/electrolytes in the water if you can't get the baby vitamins. And the yogurt will definitely help him digest what he is getting now, and help his gut run more smoothly so that hopefully soon enough we'll be rejoicing with you while you tell us about him eating grapes again.
But for now, the basics are what will heal him.
I'd even wait to worm him until he picks up some weight, honestly. Worming can be hard on them.
By the way, the water idea was great. If you DO do the vitamins/electrolytes, try to give him two things of water - one with, one without.
Be sure to keep him very near his food and water since he's kind of weak right now - that way he doesn't use up his fuel trying to go get more fuel.
I'm glad you're home - welcome home. We'll try to get your boy back on track if you'll trust us a little. /wink