Head injury vs. ADD vs. ADHD vs. something else

A 40-year-old man that I know is not able to retain much, even after working with him for long periods of time trying to teach him to do simple math, English, etc.  He has a good appearance, presents well, and has a good sense of direction while driving, and is able to make minimally acceptable answers when discussing history or world events.  He has 2 brothers who are Down Syndrome. I would like to have him tested but I don't know what for.  I am uncertain if he has mild brain damage, ADD or some other problem.  It seems when I am teaching him that he intentionally guesses one above or one below the actual answer and sometimes I think he is mimicking his Down Syndrome brothers.  He freuqently asks questions which are not appropriate such as, Why do you call Wisconsin a northern state or after completing a history course, he is uncertain of dates for the American Revolution vs. Civil War, very uncertain! He is taught over and over the difference between American grammar and Spanish grammar and is unable to show that he grasps verb noun concepts. He generally keeps a job filing or something like that for a week or two and then is moved on to another temporary assignment.  He seems as though he is intelligent and unable to
actualize even the barest success in a job or in his life.  Please assist me in figuring out where he should go for an overall evaluation.  He has had his thyroid tested and he has no thyroid problems.

[HFHS-M.D.- HG]

Dear Mary,

From the information that is provided , it is difficult to come to a specific diagnosis. It would be helpful  to know his level of education. He may be having problems with Attention or Concentration  which can be seen in several conditions such as mood disorders, attention deficit disorders etc. A learning disorder or a low IQ  is  a possibility. It is important to rule out any medical conditions that may be contributing to the situation He may consider consulting  a psychologist  for a comprehensive evaluation. I hope this information is helpful to you.

Sincerely
HFHS-M.D.

[MlCharleville]

Thank your for your help.