Does your son use sinus rinses? We were able to teach our son how to blow his nose during the sinus rinses. Having such a large volume of water coming out of his nose with gravity helped give him a really good feeling of what 'out' was. Much easier to blow out the water than to suck it back in. He still is kind of a 'wimpy' blower. Not much force behind it but it's a start.
I've also heard of putting a feather on a table and having them try to make it move with just their nose.
I've also heard of putting a feather on a table and having them try to make it move with just their nose.