Soybean meal is what feed producers in the U.S. use to up the protein % of poultry feed.I don't think its harmful in reasonable amounts. You can't feed soybeans unless processed (cooked). If sunflower seeds are reasonably available in your area, peas love them and hulled, they are a high % protein feed. If you have the time and want to take the effort to ferment their feed, fermenting makes the nutrients in grains more available to the peas, adds probiotics, and they love it.
Isn't there a vaccine for newcastle?
If it were me, I would give your cock another round of antibiotics and either check for parasites or worm them all. Good luck with him.