8Yr. Old Shut Down- HELP!!!!!!!

OK, so we finally have our new son (8 year old) at home and we are truly blessed by the fact that he has acclimated well. There have been no real issues except for one thing. On his first day of school he came home with homework. We have him in an aftercare program at school where he is able to do his homework and play till he is picked up. When we got home I looked over his homework folder and realized that he had not completed one page and advised him that he needed to finish his homework. Immediately he was mad. He argued he had finished his homework. I showed him the page he missed and he just fumed. He sat down and pouted. He did the work in extreme slow motion with heavy breathing and an ugly frown on his face. I repeatedly reassured him that it was OK, that all he had to do was complete the paper and read his reading to us. He immediately got even more upset about having to do the reading as he said he had already read it. I explained that the teachers note said he had to read it to an adult and since we are his parents he needed to do his reading to us because we had to sign the note stating that he had completed all the assignments. He finally completed the writing sheet and WOULD NOT read. I tried asking him numerous times, threatening loss of prevlidges, grounding, etc. all of which he responded with "fine, I don't care". This went on for over an hour. I even offered that he read only 2 pages instead of the 15 if he could tell me what the story was about - which he did tell me what the story was about. All of this and nothing. Finally I told him - "OK, go to bed then" and he up and headed for his room. On the way (as my heart was breaking) I asked him to come back and talk to me. I held his little hands and explained that we love him and that his responsibiity is school and we have to make sure he does his work. I asked him if the reading is too hard or if something was bothering him and FINALLY he tells us that he read it to the lady at the aftercare and he did not want to read it again. (DHUUUUU) I explained that he should have told me that he had read it to an adult and then I would have understood and that from now on I want him to do his reading at home. Since then all has been peaceful and he reads each day at home without problem. So - now we have had one other incident - last night we are eating dinner and decided to address his eating habit of stuffing so much food in his mouth that he cannot possibly fit anything more in it. We explained that he needs to take small bites (as his are bigger than a grown man takes) and eat slower. He immediately shut down - heavy breathing, ugly frown, not answering any questions, etc. We explained that we wanted him to take smaller bites as it is good manners to eat like a person, not an animal. He finally calmed down, but remained with a defensive tone. I am just curious if anyone else has had this type of problem? I feel like when he shuts down and won't answer or speak at all most of it has to do with him being mad at himself more than anything - like he is dissappointed at himself. He has a great personality and self confidence level until he believes he has disappointed us, then it is shut down time. Any suggestions on how to handle this would be appreciated. Molly