constant mucous

Hello, My 2 1/2 year old son is now entering his 7th month into a cold that has not left him since last September 06. He constantly has a runny nose that goes from runny and clear to yellow and green. It has been on and off in his chest as well, during these months. I am desperate to get rid of this once and for all, as I think it might weaken his lungs in the long term. He is otherwise quite a strong robust child, never had major problems with health so far. Only has had 2 out of the 3 shots of DPT (did not give him the 3rd as he contracted croup the night of the 2nd shot), no other vaccines given. He's had no antibiotics either. He is a late developer, walked late (17 months), still only has 14 teeth, is only starting to talk a bit (lots of baby talk though, and he is learning 2 languages..). He eats very well (though no veggies, only when mashed or hidden), drinks a lot of water and/or apple juice (diluted) has a very fast metabolism, is very independent and active. His mucous doesn't seem to disturb him too much but it worries me that it keeps going down to his lungs and he's not able to shift it. thank you for your help. Nathalie