The exact same thing happened last year, and it made the kids stressed out and miserable. I explained the text book is designed to be split into three semesters and accommodates school events, he is convinced that we will fall behind last semester, like we did this semester (We did not fall behind at all, in fact we have just finished unit 3 for both 5th and 6th today).
-Update!-
Thank you for the comments! I have convinced him to start unit 4 next semester after the summer holidays. He was worried about falling behind on schedule because of a couple of cancelled classes, and I pointed out that even with those cancelled classes we still managed to finish well before the semester ended. We'll be doing review to cement what the kids learnt this semester until summer break.
by Aegisman17