R.R.
K.,
it's hard to see them struggle. You have to be his cheerleader and encourage, encourage, ENCOURAGE! After all, it's not like he really has an option, he can't quit school in first grade! : )
Let him know that all the other students are beginning a new year, too, he's not alone, and that he is going to learn so many new things and have so much fun. Tell him the school day is longer so he can learn all those new things. It can be boring in the beginning of the year with repetition as the teacher tries to see where each student is at, but that will change. Or it can be boring because he's not focusing and paying attention to the teacher.
Tell him that he has a job, and that job is to be a good student. Tell him he needs to listen to the teacher, keep his hands to himself, and do the best he can in his school work. When he complains turn it around and ask him, "And what did you like or have fun doing today?' Run with that and put as positive a spin on school as possible. Give him a certain amount of complain time and then he's done.
If he has homework set aside a regular time to get it done each day, and if he doesn't go over his spelling words, math problems, read books, etc., to get him used to it. Encourage HIM to seek out new friends, it can be as simple as asking someone if they'd like to play together.
If you can volunteer to be a room mother. You'll only be in the classroom for parties or if the teacher has a special project she needs help with, but possibly seeing you at school will ease his problems.
And I wouldn't discipline him as of yet for the not keeping his hands to himself, but encourage him to either keep his hands clasped together in his lap or to sit on them. Let him know that part of school is behaving and that it's part of his "job" as a student.
Hang in there!