My daughter who fits the same profile came up with the following three important things about having ADHD when she was a young teen being asked for advice by other teens and parents of teens. I've also put them on a national list serve that might be very helpful for you ((GT-SPECIAL--sign up information is here:
http://gtworld.org/) and other parents have found them helpful:
1) It may seem like a problem now but it will turn out to be an advantage.
2) Its not your fault but it is your responsibility. (This is the part I would emphasize to your son--that he can't use his focusing problems as an excuse--if he wants to reach his goals in life he is going to have to find ways to overcome the challenges that his ADHD creates.)
3) Its your parents job to get you the help you need and its your job to take advantage of it.
I would also try telling him that every one has strengths and weaknesses, and both his strengths and weakness are stronger than other people's; also, what matters is not how smart he is but what he does with it and that once you have an IQ of 120 or more, IQ doesn't make much difference--its other characteristics that seem to affect what you accomplish in life. (There is research that shows this although admittedly it primarily looks at earnings.)
And you might point out that boasting about his IQ, which after all is not something he achieved--just something he was given as other kids are given exceptional speed or agility--is going to damage his friendships.
Good luck! (FWIW, he CAN overcome the ADHD issues--my daughter is now at Harvard, where she still gets extended time, and she is just one of many kids getting accommodations there. So make it clear that he can't use them as an excuse!)