Ok, I'm going to be different than your other posts... he's in culture shock and transition.1st grade is a HUGE jump from KG. I used to teach KG and they work a lot too, learning and adjusting to being in school. KG now is like 1st grade used to be when I was a kid and 1st is like 2nd. I teach 1st now and I truly believe it is one of the hardest grades for kids. I cannot tell you how many kids I hear don't like first grade after the first week or two and it breaks my heart. I want them to love school and we really try hard to make learning a lot of fun. I'm quite a goober! They eventually come around and love school again during the year, just in culture shock after the initial honeymoon period is over! What, you want me to work quietly and I have homework? They learn so much in 1st and they really get an insight in that school isn't all fun, art, centers, and songs. Learning takes a lot of time and effort and 6 year olds aren't always mature enough to handle it yet. It will get easier and he will adjust. There is so much they have to learn in this one little year. My goodness, just reading alone is a lot of work. At the beginning, we transition them slowly into the work load, learning, and homework but it's going to happen- spelling tests, writing assignments, math worksheets, and more. He's just used to having more freedom, choice, and fun. Watch him carefully to see if there's an issue with the teacher or a student, but if not, give it time (perhaps the first six weeks). If he's still really emotional, talk to the teacher.
Oh, as for the behavior.. my daughter's friend put it this way in 1st grade, she said that all day long she is trying so hard to be good and follow rules that she just can't keep it in all day at home. haha! He's tired and needs to let off some steam. He can't do it at school because he doesn't want to get in trouble and he knows that you're his mom and will always love him no matter what. My kids (9 and 7) are awful after school from about 4-6, I give them some down time in their rooms or in front of the TV for part of that, sometimes separately and that usually helps. Make sure he's also getting plenty of time to play and just be a kid after school. :)
Ok, that was long. I hope it helps! Good luck!
Update- I forgot. His fingers hurt because he is working hard and building up the muscles in his hands and fingers. They'll get stronger. :)