Hi there I'm in Scotland, our son's treatment is west coast (paisley) but I'm sure that it will be the same course of treatments and tests. IT does sound very much like CF with these symptoms. You can get a negative result with a sweat test, so it can not be taken for granted, a genetic test is and will be the next course that should be taken.
Did they not find anything in the guthrie test (heel prick)when your son was born? CF has been included in the guthrie test for the past 5 years in Scotland. That is how our son was found to have CF.
Any question you can think of or need to talk to just pm me and either myself or my wife will get back to you.
Craig