Hello concernedforhim and welcome to Healing Well!
I would definately request that the sweat test be given to your son. Make sure it is done at an accredited center. At 22 months, I would think that your son is old enough to have a sweat test. There are other tests that your doctor can order so that all the pieces can be put together for the "puzzle" on a diagnosis. Some that I can think of are throat cultures to see if he is culturing any cf bugs, fecal fat test to see if he is pancreatic sufficiant, sinus ct's to see if he has polyps, full panel/gene dna testing. There is even a nasal potential difference test (not recommended for young children).
I hope you get some answers soon as I know how difficult the waiting and wondering can be.