I have a 10 years old son, he loves to be with his daddy, he is always telling me that
He wants to live with me. Can somebody help? I just want to know how old he must be before he decides which parent to live with??

He can decide on his own at 18.

Somewhere between 10 and 12, the local courts will allow him to testify where he wants to live in court. So you will need an attorney, file for a change in the custody order.

In it, you will try and show why he is better off with you and the other parent does the same for thierself,
Then the child will talk to the judge, often in private in the judges chambers. But may be questioned by both attorneys within limits in court