I have a facebook account. My son wants to sign

You must first log out your own account from facebook.

If you wanted to create facebook account for your son, you must create him his own mail account

When he has his own account, go on to facebook site and sign him up using his own mail account. Just remember facebook has age issue, which means that if your son is below 18 yrs old, he will not be allowed to have a facebook account. If you wanted, you can change the year of his birth date, make him more older. In that way he can have his own account.

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Choose Account in the top right corner then logout
open facebook site again.