Good to hear that the sling/hammock is making him comfortable and he is accepting it. Do encourage him to walk a little each day out of the sling and give him a target to move towards..... like putting him outside a short distance away from the girls' pen and letting him make his way towards them. Regular short, focussed exercise like that may help him to re coordinate his legs over time. Vitamin B will benefit him regardless of the cause of neurological problem... ie Marek's, toxin or even liver issues. It is water soluble so any excess can be excreted and the body has a reasonably high tolerance level for it. I don't know much about selenium, so can't advise on that. Active charcoal was something that I was about to suggest if toxins were the issue. The important thing is not to put him off his food by adding things that he doesn't like, so try it and if he won't eat it, don't persist, but go back to what he is enjoying. It just stresses you and him, trying to get stuff into him that he doesn't want.... I've been there!
Don't worry too much about him drinking water if his food is sloppy. They don't generally need it if they are getting it in their food. Dry food requires them to drink in order to digest it, hens laying eggs need plenty of water because eggs are made up of quite a lot of water, in hot weather they drink more to cool down but if your guy is getting water in his food and is not too hot, I would not be concerned about how much he drinks or doesn't, as long as he has access to water if he wants it.
Good luck with him
Barbara
Don't worry too much about him drinking water if his food is sloppy. They don't generally need it if they are getting it in their food. Dry food requires them to drink in order to digest it, hens laying eggs need plenty of water because eggs are made up of quite a lot of water, in hot weather they drink more to cool down but if your guy is getting water in his food and is not too hot, I would not be concerned about how much he drinks or doesn't, as long as he has access to water if he wants it.
Good luck with him
Barbara