Kath

My phlegm was also thick that the baking soda/salt rinse did not help at all. What worked for me was plain old seltzer water in the cans. The large bottles did not work because the fizz was lost too quickly but one can was just the right amount to swish and gargle and spit about three times. And I needed all three to clear out the phlegm. A case of 24 cans was about $4 and I bought the generic store version.
I still had to get up three or four times a night but at least I was not choking all the time.
Another hint is to get one of the BIG travel coffee/drink mugs and keep it in the car so that your brother can gargle and spit while on the road. I bought one with me to the radiation waiting room and right around the 5th week had to upgrade to a "spit bucket" - one of those cylindrical coolers to accomodate the volume.
Best wishes Please assure your brother that while such thick phlegm plagued me during and even after radiation, but it did get better.