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.
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 ....