I need some advice.

My 10 yo son really wanted a polish chicken to add to our flock this year. We have looked everywhere for one. We kept missing them at the feed stores and no one had any on Facebook.  Yesterday we stopped by a feed store to get some layer feed, and wee stopped by the chicks. They had one polish left. He was so excited. When I looked at it, I could tell it wasn't doing very good. He begged for it and said he wanted to try and save it. The lady gave it to us at a discount and I got two other chicks for companions.

It won't eat. I have been successful with water. I gave it sugar water after we got it and it perked up. I did see it pick up two pieces of crumble, but I don't think it ate them. I have tried cooked egg yoke, and then raw egg white. It didn't eat those either. I mixed electrolytes, water and chick feed into a mash. It won't eat the mash, but drinks the water from it.  I have been holding it on my lap, under a heated blanket. I let it drink the water from the mixture every time it wakes up. After it drinks, it gets sleepy and goes back to sleep. It has been having bowel movements. Last night I held it for as long as I could and then I put it with the other chicks to sleep for the night. It made it through the night. It's acting the same today as it did yesterday. I offered it the mash again this morning,  but it only drank the water and went back to sleep.  I don't think it's going to make it, but I want to give this guy a chance. Is there anything else I can offer it? It needs to eat to survive.  

I explained to my son before we bought the little guy, some chicks never thrive and it can be for a number of reasons.  I wanted him to be prepared for the worst-case scenario.  I'm cautiously optimistic since it made it through the night. But if it doesn't start eating, I don't think it will make it.

I need to know what people have had success with, so I can try that as well and give this guy the best shot at life that I can.

Any advice it welcome.

Failing to thrive chick