yeah, have fun with this one.
first question; does he have a cold, allergies or other nasal obstruction? thats a main reason for kids chewing with their mouths open.
also, kids learn best by example... soooo if you routinely chew or talk with your mouths open your son will too! its a proven fact! :P
also, a more personal question, do you (or anyone else)smoke around your child? smoking is very unsafe for young kids lungs (which im sure you know, i dont want to guilt trip you) but also it is SO easy to plug those little noses up with smoking. kids breathe twice as fast as adults, so not only do they get more into their bodies and lungs, but they get stuffed up twice as fast. soooo... caution there.
:D
anyway,
just keep showing by example. this also drives my husband crazy, and i had a child care kid who did this for over a year before he got the hint. mom smokes so him and his sister are ALWAYS stuffed up. we just have to not sit at the table with them, however, now that they are almost 5 and 5.5 yrs old... they do much much better, with some occasional reminding.
getting mad isnt going to help, so just be patient, and keep showing good table manners. :D