My dog has lots of boogies...why?

hes 3 years old and has been a mucous mess since day 1. he sneezes and hacks and yaks and blows boogers around the house that can be like 2 feet long! gross.. I've spent unbelievable hours researching and ridiculous funds at the vet, with no answers except allergies... no medicine has worked, some make him puke..add that to the mess :) ..his boogies are thick and greenish yellow..stick like glue to everything he flings them on... oh and yeah he's a huge German shepherd.. so big messes...yuck, yuck, its driving me crazy, we cant sleep, and no one likes my poor puppy because he thinks everyone is a giant Kleenex... coincidentally, he never sneezes for the vet, and his nose is perfectly clean, so they check his lungs, tell me hes fine, charge me 300 bucks a pop and send me home. yay. help!