As someone with Marek's in their flock I have cared for quite a few. Some make it, some don't. Some go from being totally fine, to unable to walk in a day and floundering around on their side. Then a few days/week later they are fine again. Others take months of TLC to make a limited recovery. The things that have had a big impact on mine have been sunshine, grass and the company of other chickens, plus the stimulus to forage. On warm sunny days I put mine out on the grass in a 4ft cage and scatter scratch in and around the cage, so that the other chickens forage around the disabled one. The illusion of competing for food encourages them to move around and that helps them work through the paralysis. You can also achieve this by encouraging them to walk towards you for their food. Start with just short distances a couple of times a day. They will flounder about and look awkward, but it is all part of the process. Slings are good for keeping them upright which helps digestion as well as preventing them soiling themselves but as drumstick diva suggests exercise therapy is also important.
As regards suddenly not wanting to drink the water with the vitamins in it, that would make me think there is something amiss. Perhaps the dose is too strong. It's important that birds get good nutrition, but don't push the supplements to the extent that they get sick of it and start refusing to eat or drink as that defeats the object. I went through this when trying to treat them with turmeric and black pepper. I started getting stressed about getting it into them when they got sick of it after the first week. I realised that keeping them happy and stress free and eating enthusiastically was more important and beneficial than the turmeric, so don't get too hung up on the vitamin supplements. Have a break and then come back to them.
Good luck with him and I hope he continues to make progress.
Regards
Barbara