Thank you very much, both, for your helpful comments :) Sandman2, I will read the helpguide, and allmymarbles, I had never thought of sleep apnea, will check that and also the dietary advice. (He does seem to need less sleep than his siblings, whatever that means). Re. language acquisition, English (the language he learned to understand) is never spoken in his presence, but he hears it on the radio (songs) and on tv. His language is Icelandic. I also have a daughter who lived abroad for ten months as a child and did not pick up the language, she still finds it difficult to learn languages (at 11) and even understand if people speak fast or unlike what she is used to in her own language. She is diagnosed with ADHD and anxiety disorder.
Check his diet for high sugar or for sugar substitutes like aspartame. Check him for sleep apnea.
By the way, as for understanding a foreign language without being taught, young children learn just by hearing the language spoken more or less regularly. My three older children were bilingual from the earliest age by exposure only. My fourth child was trilingual from the time she started to speak. This is not at all unusual given the environment. Oh, and when my son was nine years old and went with a school pal to the pal's family in Israel, he returned home one month later cursing in Hebrew. And, even more interesting, when we left child number four (who was about 18 months at the time) with an Israel couple for the afternoon, she spoke what we thought was gobbledegook for the rest of the day. But my son said that she was mimicking Hebrew sounds, but without any recognizable words.
In case you haven't guessed, language fascinates me, and the process of learning language fascinates me even more.
Well, he apparently is quite intelligent. And he also is only 5. It may just be a combination of intelligence, immaturity, older siblings, and lack of consistent training that is causing these actions. However, he does also have some of the ADHD symptoms and
you mentioned that ADHD runs in the family. So on the possibility that ADHD is causing some of these problems, let me recommend a site that has a lot of really good information. Maybe, the best I have found so far for children with ADHD. Its
http://helpguide.org/mental/adhd_add_signs_symptoms.htm
Hope this helps. Best wishes.