M.G.
some times you just have to punnish because lets face it, he is just not listening and taking notice of you.
my son is 3 he knows and does sit up straight elbow off the table (once in a while he will actually remind us of that) and he chews with his mouth closed.
how ever this did not come naturally, i get very grosed out about bad table manners.
the way we delt with it was after trying the nice approach, we played hard ball.
i told him if he didnt eat properly he could eat on the floor with the animals and on the days that threat didnt go over well i just took the food away and told him he could eat at the next meal when he would use the manners that we had taught him.
this lasted for a bout 4-6 meal times and touch wood we havent had a problem since.
sometimes being a parent means you have to be the adult and not just the peace maker, friend.
now you tell you hubby to get on the same page as you or eat somewhere else because you have to start training them from the get go because other wise you have to spend the time de-programming them and then re-programming them and why would you put your child and yourself threw all that. in my opinion that it just really cruel.
hope this helps.