If your grandchild suffers from anxiety (whether severe or mild), what you are seeing is very common. Children with anxiety issues usually find that after a break - whether Christmas, Easter, summer or even a three-day week-end - great difficulty in getting back into the "school" routine. They have gone from a relaxing and safe time at home to a more stressful and perceived unsafe time at school. Just be patient - treat his going to school as a very normal thing and try not to give attention to his fear issues. I'm assuming that he does well in the classroom after a short time of arriving at school. Hope this helps ....
I am wondering if he had a new teacher or anything has happened in his life that is differant and would cause anxiety, you say it is sudden so look to anthing new , family siblings ,Loss, how does he get on at school, have you spoken to his teacher for feedback from her.