Something I did when my son was younger and acting exactly the way you describe (usually in the dead center of walmart or something) was to print up business cards.

"If you are reading this card you have expressed concern or opinion reguarding my son. My son has a condition called Early Onset Bipolar Disorder. Please refrain from offering advice and/or comments as they set back the behavior plan put in place by his medical staff. If you are interested in learning more about this disorder please visit bpkids.org. Thank you for your understanding."

Sounds corny but it really works. I wouldnt even say a word. When some onlooker would say something like you describe I would simply hand them a card and continue on with what I was trying to do. they would usually roll their eyes and slink off, a few would appologise and smile at me, and I can only remember 3 times that anyone continued to offer comments after I gave it to them, and I have passed out HUNDREDS

Something to keep in mind with the "beat his butt" comments is "You don't beat a seizure out of a child with epilipsy, and you don't beat the bipolar out of a cycling child." With age, proper medication, and more patience than most people will ever have to muster up, it does get better. You have to remember that proper medication is the only way he can improve. If he isn't medicated the right way the dots are not going to add up for him and he will not be able to feel empathy or remorse OR be able to learn to feel those things. But he will get there.

I feel so bad for you. I remember feeling as helpless and dejected as you are right now. It was hell, and no one can understand the endless day and night war that goes on in your house. I also remember what it was like hating myself because sometimes it was hard to even feel love and compassion for my child who could not help how he was acting.

It does get better. You just have to hang in long enough to find the right medications and doses. (Zyprexa was the only drug that ever worked for my son)