S.H.
When I subbed at my dh's school (she's in second grade) for a week, my ds went to school there, too (preschool) for that week. I thought it would be hard on him to not be with her because they sleep, eat, play, etc. together. SHE was the one who cried when it was time to go in from recess and his class was coming out. She had been looking forward to hanging out with him in school. It was so sweet!
I bet your boys will be fine. It might take a day or two, but school is so full of activities that he won't have too much time to lament the missing sibling. Kids also surprise us so much with their independence when we are not around.
I don't think it is possible for kids to be too close. I think it is the greatest blessing a parent can ever hope for. You can go to your grave knowing that they will look after each other. That's what gives me the greatest peace in my heart.