Hello and thank you for consulting JA,
The background information is very helpful. There are a few possibilities which we can start to discuss here but will need further evaluation by a child psychologist in person.
The first is related to adjustment to 1st grade. This is a common problem especially for children who are high-strung or anxious. The demands of 1st grade are more that they are used to and it sounds like his struggles are apparent to him. In other words, he may be frustrated by his struggles. If it is just an adjustment, then it should get better with time.
Since he is not melting down at school, it means that he is controlling this negative energy there. But it is "at a cost" because it requires energy and is fatiguing. Then, when he gets home, he let's it all out. Home is the safe place to do so.
Some of the behaviors you describe are symptoms of Attention Deficit Hyperactivity Disorder. That is where the psychological evaluation applies. Here are the 18 behaviors of ADHD: http://www.behavenet.com/capsules/disorders/adhd.htm
As far as further intervention, since reading is such a high priority in your family, you may be able to benefit from therapeutic books published by Magination Press. Here are a few that may apply but do look at the website for the many selections:http://www.apa.org/pubs/magination/441B037.aspxhttp://www.apa.org/pubs/magination/4414681.aspx
You might want to have a discussion with him about how uncomfortable his feelings seem to be for him and that.... just like if he had a stomach ache...you want to help him feel better. Tell him that reading one of these books together might help and let him choose. While you are talking and deciding about books, if some theories begin to evolve about why he is upset or angry (something at school, jealous of sister) then target that subject when you choose a book.
I hope this has been helpful but please feel free to ask a follow up question.