If it is interfering with his life (and how could it not?) then you might consider having an evaluation for ADHD, inattentive type. A developmental pediatrician, or a psychiatrist/w a nueropsychological evaluation can tell you for sure. You could also see a neurologist, but we have not had much luck with them.
Loosing track of thoughts is one classic symptom of inattentive type ADHD. Thoughts in the brain are carried from brain cell to brain cell as electrical impulses, and because the brain cells do not touch, we use neurotrasmitters (chemicals) to carry the thoughts accross that tiny space (called a synapes.) If the receptors for the neruotransmitter are damaged or the production of neurotrasmitters is reduced, that person looses track of thoughts and has ADHD. That is one reason the medications for ADHD work so well, they assist the brain in more reliable thought continuation.
The medication is a tool that will help him access all the other treatment avalaible to him like therapy and educational interventions.
If you are not exaggerating, and it happens all the time, he is probably really miserable. I would get him an evaluation so that he can get treatment. While there are other issues that could cause this, ADHD inattentive type is the most common, but find out for sure by asking for an evaluation based on his symptoms.
Good luck,
M.