You could try saline in the nose which of course he won't like but works pretty well. Since you mentioned the nebulizer I will comment that my son is on that too and the albuterol seems to help (requires prescription).
Lastly, I agree with the post about avoiding dairy products. We just recently switched our almost 3 year old son strictly to rice milk since he has chronic ear infections and lots of mucus. He has had tubes put in twice now and we are trying to avoid a 3rd surgery. The rice milk along with chiropractic treatments has seemed to help. I was concerned at first about switching him to rice milk because I wanted to make sure he got enough calcium but it turns out the rice milk has the same amount of calcium as cow's milk. Unfortunately it is pretty pricey but you can get the best price for it at Trader Joes (I think about $3 for 1/2 gal vs $4/pint at cub).