I am so sorry about your baby chick. We all were so excited for him. That is heartbreaking news. I wonder why he got worse. My sympathies to you. At least you got some experience from this for the future.
That is just too cute. I think this little chick will make it with your help. Try the boiling egg mash again. It will be good for the chick, he needs nutrients to get strong.
Did you try to feed the injured chick a yoke from syringe? It saved my chick that was badly injured and wouldn't eat. I was given advise on this site to boil egg and mash yoke with water. It is very nutritious and will give the chick all needed vitamins.