Well, it has absolutely nothing to do with cancer. Sheesh... The stupid things these MDs communicate to us.
It's more likely your son is hyperthyroid. Graves' disease is the most common cause of hyperthyroidism. Its symptoms include some that would be quite like ADD.
The thyroid uptake scan is an outdated diagnostic test for Graves'. Make sure he has the latest, most definitive one - the TSI antibodies test. Any titer of TSI antibodies is abnormal, whether it's within normal lab range or not.
Consider that this doctor may not be the best to treat hyperT/Graves' disease, based on the "cancer" comment. Be very wary if this doctor or any other one wants to treat him with RAI ablation before trying anti-thyroid medication and a beta blocker med to lower excess hormone levels.
If the diagnosis turns out to be Graves', educate yourself fully before agreeing to any treatment other than anti-thyroid drugs. The book and author who wrote it, Graves' Disease: A Practical Guide
, are excellent resources.