Ours was strawberry flavored. The first time M did it, she was 12 and refused to drink after a certain point. We had to bribe her and then she agreed to drink the rest. She was not strapped down for the test, but maybe that's because she was older and was able to lie still.
They said they prefer that kids drink it and don't use NG tubes because they like to see if they're able to swallow properly or if they have GERD etc. However, if he absolutely can't drink they'd probably use an NG tube.
Definitely bring those extra clothes! Also, have him drink a lot of water after the scan, to flush the barium out.