Couple the poor social awareness with the impulsivity that is a part of ADHD and you've got a kid who says the wrong thing at the wrong time without thinking. That's quite a devastating combination. Your child may need to hear the message about not being rude to others many times until the behavior becomes automatic, so, you do need to have this discussion with him again and again. You might want to vary the way you send the message. Consider using puppets or role-play and act out interactions between characters. Watching parts of TV shows or videos with the sound off so that your son can learn to "read" non-verbal cues can also be helpful.
However, you and the teacher may need to do something a bit more intensive to help your child learn these important skills. It's not really wrong that there are consequences for negative behaviors, since your son needs to have feedback from the world around him. However, research tells us that a positive point system is much more effective (and better for the child's self-concept) than a negative (take away) system.
Here's how it's done. You and the teachers should identify a specific problem behavior (such as saying disrespectful things to others). First, the teacher should determine (by recording behavior) how often this behavior occurs in the classroom. If, for example, it happens two times per hour, then the goal should be to try to reduce the incidence of this behavior to one time per hour. The goal should be related to the actual average number of times the behavior is seen during a baseline period of two to three days. Then the goal should be shared with your son. You might create a chart with some symbol for the behavior (a line drawing of a face, or a Polaroid photo of your son with an X drawn over the mouth, for example). Then you need to determine what items or activities are rewarding (and acceptable in school) for your son. If he is able to meet the goal, he gets the reward.
This is called positive reinforcement, and your son's teacher or the special education teacher should know how to set this up. In this way, positive goals are rewarded, and this should result in improved behavior. Once he meets the goal, then you'll want to raise the standards (no such behavior every hour or less than two episodes during the morning or one per day, etc.). You may also have to vary the rewards or reinforcers. You should also use this plan at home, so there is consistency. Ask the folks at the school to help you set this up.
You and the teachers need to understand that the behavior may be a part of the Asperger's or the ADHD or a combination of both. You also need to consider that it's neither. The child may have just gotten away with such behavior at home for a long time, and now it's carrying over into school. I don't mean to imply that you haven't been doing a good job at home, but I do know how hard it is to be consistent with kids whose special needs can quickly wear a parent down. At any rate, the plan I've described should help at home and at school if you stick with it.