There seems to be some controversy about whether dairy products cause an increase in mucous or not (in the general population). I think one's subjective experience tells the tale. If you consume dairy products and have noticed more mucous when you do, there's probably a causative link.
In Gordon's case, he's been having the "milkshakes" (fortified milk, diabetic Boost, whey protein powder and sugar & lactose-free ice cream) for over 4 months without having any discernible increase in mucous, so I suspect his lung problem is that he's aspirating something. I wonder if there's something about the texture of the milkshakes that might induce more aspiration. However, it certainly wouldn't hurt to try some soy milk to see if it makes a difference. We tried soy yogurt earlier on and he wouldn't touch it, but maybe he'd be more willing to try the soy milk now (I've seen the Silk around, so will give it a go).
Thanks again for your help, Anne