Sue, my children lost their father to cancer at ages 8 and 11. Although I made certain that they both received counseling services both before and after his death, they both have encountered many difficulties in dealing with their grief over the years.
One of my daughters was also diagnosed as ADHD, however, around age 16, the diagnosis was ammended to Anxiety Disorder, then around age 18, Bipolar Disorder was added to the mix. Intervention and treatment/therapy are the best help, but keep in mind that once is usually not enough. Your son may need periodic help.
Ray is sooo right--be sure to take care of yourself. Your son will need your strength in the future. Be prepared for the unexpected, and devise contingency plans if the future has bumpy areas. Seek out support from both professionals (school, church, Hospice therapy, etc) agencies, and from those here on CM (or other organizations). God bless you.