Your 13-year-old has serious problems and needs a doctor's help. I was 7 and 98 when I had these problems, and the doctor took care of it.
People with ADD benefit most from structure - they need a set schedule, even on days off, where they get up at a set time and study at a set time. Focusing exercises help also - with smaller children, you can give a star or other token every 10-15 minutes of on-task working, then allow them to trade tokens in for a reward. Remember that you need to set smaller time increments with ADD students than with non-ADD students.
Yes. OCD (Obsessive Compulsive Disorder) is an anxiety disorder that involves not only anxiety, but the hyper-focused need to perform a certain - usually unnecessary - task to feel 'right', which is also seen in patients with ADHD. Patients with either ADHD or OCD both share the common sign of being obsessive: Someone with ADHD might be almost 'obsessed' with something, whether it be a school subject, a computer game, a book, or anything else, and wouldn't feel good or satisfied until this obsession was 'fed' (by playing the game continuously or learing everything about the ceratin subject) - and people with OCD are very much like this as well.