My son does learn those skills (sitting properly at a table)...every day at home. As far as rewards/consequences...yes, that works great for our NT ("nuerologically typical" or "normal") kids (we have 5 kids). We also do teach a bit when out to eat. As I said, he is expected to stop playing his game and look up at the waiter when he orders. He is expected to stop playing if someone is speaking to him and he is expected to stop playing once his meal arrives. As for the rest of the time...try going into a place that makes you a uncomfortable: put on some strob lighting, blast a stereo, have people all over, a TV on, fill the room with overwhelming smells, etc and try to sit and carry on a polite conversation. That might give you some idea of the sensory overload an ASD kid feels. Right now the DS helps him cope. As for looking down the road, he will always have a spectrum disorder. As he gets older, he will learn new/better coping skills (he already has).
Jess