As the Mother of another Webelos Scout with ADHD, let me offer a differing opinion. I was my son's Den Leader for 2 years and he drove me crazy at every meeting. Other kids would have some of the same behavours (den of 14 - 4 with adhd) and I could ignore them, but when it was my son, i went nuts.
Do the parents hold positions in your troop? If not, perhaps it would be better for him if the parents didn't hang around the troop/patrol meetings. I only say this because I have moved from Den Leader to Assistant Cubmaster and my son is doing better in his Den Meetings without me around! Bad for my ego, but great for his! Is there an Assistant Scoutmaster or an older Scout that could act as his "Buddy" and try to keep him on course during the meetings? This method has worked for my son.
As stated before, please don't give up on this boy. Many people don't realize how difficult is can be to work with children with adhd. Many people don't even recognize it as a valid diagnosis. This is a real issue for these children and they (and their parents) need all the supportive help they can get. Perhaps you can speak with a physician or psychologist who has some experience with Attention Disorders and get some advice or insight. There are many books available that offer stratigies for correcting these types of behaviours. I'm sure your local library has many. Check it out, you will not only benefit this particular boy, but every child you deal with in the future.
Good Luck and thanks for your concern. I hope my son will have a Scoutmaster who is as willing to help him as you semm to be to help your Scouts.