As the child was already in CBSE board from Grade I, he will find it convenient in Grade XI & XII also. Hence as far as his studies is concerned you need not worry. But the only problem is he will have to spend extra time to take down the notes of these months and to memorize it. As he is a grown up boy it will not be very difficult for him.
As to change the school, try speaking to any near by CBSE schools and if they are ready to accommodate one more child, then accordingly speak to his current school and do the needful. Let the school authorities know your situation so that they will be ready to let your child in. If in case you fail to find a good CBSE school for your child, let him somehow study in the state board this year and change him next year to the CBSE school.
For your child to adjust with the state board, give him tuition and let him do combine studies with his class mates. If you know any child from your neighborhood who is in your son's class, then let him make friends with that child so that they can learn together as it will take time for your child to make friends in this new school. Once he feel himself comfortable, may be he will be able to study better in the State board.
"Do not give up, things might not favour you always"