I don't know if this will help you or not, but in my den we have a code of conduct. The boys helped to write it and agreed to follow it. Also at the beginning of the year I gave each parent a letter stating what was expected from the boys and what would happen if they could not follow the rules. If the boy has to be corrected twice the parent is called or asked to take the boy home. The next time he is asked to leave he is not allowed to return untill the boy and his parents meet with the den leader and the cubmaster. I have two ADHD boys in my den, while they do require extra handling I haven't had too much problems with them. I do not lower the standards for them, I also be sure to praise them when they do behave.
You also might try asking the parent to just drop the boy off and not stay, some boys act better when their parents aren't around.