Do what you can to get him hydrated. Can you get some poultry vitamins and water into him then see if you can get him to eat some wet feed?I don't think he has brain damage because if he did he'd be falling over a lot. As it is it's like he's in roosting mode so he doesn't want to eat because its like he's roosting. He just wants to sleep and when you put him in his box he tucks his head under his wing and sleeps. It's been a couple of days now and he doesn't seem to be getting worse, if anything better because he is slightly more alert and struggles when picked up. He can stretch a wing without falling over and shake his feathers, walk around but he just doesn't make any sound or eat or drink. I just hope he comes out of it soon :-(
You may be able to hold him like some do when assisting birds with Wry Neck (video below), if not then consider tubing fluids into him.
Since the Silkies do have a hole in their head, a bump on the head can cause severe injury. I don't know of anything else that can be done but offer supportive care. Sometime an anti-inflammatory prescribed by the vet may help.
Tube Feeding: https://www.backyardchickens.com/th...res-under-construction.1064392/#post-18827506
Silkies Skull:http://www.browneggblueegg.com/Article/HoleInHead.html