My fiancé's brother who is in year 10 wants to be home school but I don't know how to go about it and is their a certain age u have to be. I'm 18. And in the uk. Thank you

Is your fiancé his legal guardian?
(You aren't married to him so would not be involved.)

He is in our custordy full time and we both look after him. My fiancé works and I am the only one who could teach him from home.

You may help fiancé look after him but you have no legal authority to do anything.

So while you may "help" the legal court appointed guardian is responsible for the education and only that person can decide to home school.

It is normally easier to use a online home school program.

UK Homeschool - Lets Homeschool

Home Schooling and The Law | Homeschooling