E.P.
I see this happen with my children's friends now at 10 years and 13 years. I just continually have to remind my children to do this and be an example. And comment when they do have good table manners - positive reinforcement works very well. Also, sometimes we play a game I thought I made up but I've seen this in a magazine since. "The penny game" Each person around the table gets 5 pennies. If you see someone do something unmannerly, you may call them on it and take one of their pennies (without yelling or pointing because this can be bad manners too - and their penny can get taken. Yes, my kids can get competitive with this!) Elbow on table, hands on furniture, eating or shoveling with fingers, open mouth with food, burping and not saying excuse me, grabbing something with asking to pass it and not saying PLEASE and THANK YOU. I throw in a few, blatant, bad manners and the kids think it's funny to catch me. When my kids were younger and had friends over for lunch, the friends would always ask to play it. Just know that you will get tired of playing it if you do it at every meal.
Regarding open mouth... If his nose is stuffed up, this will happen. Also, if my kids talke with food in their mouth, I tell my kids, "I want to hear what you have to say so I'll be patient and wait for you to finish chewing". (then they chew really fast!) Good luck!