Okay I'm updating again in the hopes it helps someone in the same situation some time.
I honestly didn't think my boy was gonna make it, he was so bad. Then I got him on vitamin B complex, vitamin E, vitamin A, electrolyte water, and a feed mix of scrambled eggs, tuna, chopped Brazil nuts, chopped leafy greens and sunflower oil with his grower pellets mixed through... He improved so amazingly. It was quick at first, but it's slowed down a bit so I hope he continues to improve even if it does take a while.
I made him neck braces out of toilet rolls (which really seemed to help), and my girlfriend had the great idea to put him inside our huge netted off veggie garden. He did so well eating and drinking by himself when he was able to run around, scratch the dirt, peck at the plants and sunbathe. He even made me chase him around the yard and climb under the hedges to fetch him from one of his favourite dust bathing spot when he managed to get out of the veggie patch. He lost his place in the pecking order and couldn't get along with our other cockerel and 2 pullets, but he had so much spirit back. He started not wanting to put up with me feeding him vitamins, which I take as a good sign, hahaha. He actually seems almost exactly like his old self, then occasionally has an episode, but they're getting less severe, shorter, and less frequent. It's usually worse at night though, so I propped his neck up on a rolled up towel so he could rest better.
Here he is wearing his neck brace one evening. It doesn't even truly show his improvement coz I didn't manage t get photos of him running around outside during the day, but as you can see, he's keeping his chin up =)
And! Merry Christmas from my special little guy!