My grandma does the same thing, its very loud, but she isn't doing it on purpose. When the noise happens its involuntary. You can control it if you take slow sips, but general drinking of liquids is just uncontrollable. Not everyone has this problem.
For the people saying that swallowing loudly is annoying or poor etiquette, for the love of God get over yourselves! Some people cannot help it and are just as conscious of it as you. I find it ridiculous that people like you make others feel uncomfortable to do a natural thing like eating or drinking so as not to disturb/annoy others. Utterly shameful!!
maybe my mom has this-- ugh so annoying
Have him try this:
Take a swallow (of a drink or a bite of food)
Exhale first
Then swallow
And see if that helps.
I think what loud swallowers do is
Inhale
Swallow
Then Exhale (causing gasping or loud gulping sounds)