My son did not go to school last Friday due to fever how do I write a letter stating this?

Exactly below as follows:

To whom it may concern,

My son (put his name here) was absent from school the other day due to a high fever. He will be willing to do any missing assignments and work needs. Thank you for your understanding.

Sincerely,

(your name here)

It's important to contact the school as soon as you possibly can if your child has had a day off school for any reason. This is so the school knows that your child is going to be absent, and can make a note of it in the register.

If your child has taken a day off school due to illness, you can write a letter to the school to explain why by following the format below. 

Absence letter format

For administrative purposes, your letter should be laid out in a way that's clear to read, and contains enough information.  A casual letter is usually accepted but isn't really optimal.  The best bet is to follow this layout:

1. You should put the school's address at the top of the letter.
2. Below the school's address you should write your own address.
3. The next thing to write is the date of the letter.
4. Now you should write "Dear Mr/Mrs (insert name)".
5. The following information is the most important.  You should now write the main body of your letter.  State the date that your child was absent from school, and the reasons why.  It's also a good idea to ask for any relevant catchup work to be provided to your child.
6. You should now provide a contact number for any questions the teacher may have.
7. Sign your letter, and don't forget to let the school know the name of your child!

That's all there is to it.  Simply make sure you provide enough information and you're good to go.