Why you need him? (This aspect is very important, because your child who suppose to go to school, has to be at home for your care)
So, you need to give a solid answer, where you will be telling about the genuine reason why you need him/her.
Obviously, no parent would want her child to be absent from school, so the school is going to consider you, as you as a parent is going to request the Principal.
Here, I have helped you through the following post
Requesting to allow my child to take care of me for 3 days due to surgery
City Montessori School,
Subject (Requesting to allow my child to take care of me for 3 days)
I am Mr. Sharma father of Anil Sharma student of Class 10th A, requests you to kindly allow my child for my care for 3 days starting from tomorrow i.e 16-04-2018 till 18-04-2018
The doctor has advised my complete rest where I have to be under extreme care. Since, he is my only help for the time being, so I would request you to allow him for three days only.
He will resume school from 19-04-2018
I will be highly obliged,
Father of Anil Sharma
12/56, X Road,
15th April, 2018