Ugh! How revolting!!!! Just thinking about it makes me feel blech, let alone if I was trying to eat in front of someone doing it. He sounds extremely immature and insecure. The part about whispering at someone whilst looking at them - rude, rude, rude, rude. Most people have hopefully outgrown that by the end of primary school.
I'd bring it up with other group members (how many of you are there?) and if some or all of them are just as sickened as you are, then definitely say something to the mother. (In fact, even if none of them are bothered by it, still say something to his mother). If she refuses to say something to him about his behaviour, I'd say something myself, very directly and without humour. Point out that most mature, secure adults don't feel the need to mark their territory/behave in this way. If he continues to act this way, which he probably will, then get up and walk out. Perhaps have an agreement within the group that if this happens even one more time that all of those who are bothered by it will walk out.
Also, if the mother refuses to say anything, or doesn't even see a problem with it, then stop asking her to the dinners.