You are doing great to try getting Nutrical, Pedialyte and kitten formula into him. Keeping him hydrated is a very important part of nursing him through this infection.
One concern has to be that this little one could be infected with a bad virus, like feline leukemia. He can be tested for it, but you have to get a few drops of blood, which can be difficult in small sick kittens.
A nebulizer is a good idea. You also could put him in the bathroom after it gets hot and steamy after a shower.
You can get children's nose drops, called LIttle Noses, and put a small amount in his nostrils to moisturize the nasal lining and try to thin out the thick nasal secretions.
Keep him warm, keep syringing water and Pedialyte into him. You can get some strained meat flavor baby foods
and add enough water to syringe that into him.
If you are going to save this little one, it will be with your good nursing care.
Let me know what else I can help with or answer.