It may be that your son is having more difficulties because the curriculum is getting more challenging. The beginning of the year could have been the "honeymoon" period, and now things (yes, in 2nd grade!) are starting to get more serious. There's more reading and writing and a greater need to sit still for longer periods of time. These requirements may be placing your son under increasing stress and he may be letting you and his teachers know it by his behavior. Remember that with kids, behavior always means something. The key here is figuring it out. What happens when your son talks back? What kind of a reaction does he get from his teachers and the other kids? Does this behavior seem impulsive -- is he able to control it, or does it just "pop out?"
If your son genuinely feels bad when he misbehaves, he might just be having problems controlling himself, which can be related to the ADHD. If being punished doesn't work, then this is even more likely. It's important to remember that kids don't like to misbehave! They like to get along, do what's fun, and have good feelings about what they do. If this isn't the case, then someone needs to help you get in there and try to determine what needs are not being met, and to set up a plan of action that helps get him what he's looking for (which is certainly not punishment). It may be more structure, it may be that he's bored and needs more stimulation, he may be seeking attention, or he may be angry or scared about something. Talk to an experienced child psychologist about this. And don't wait.