I have to retain my son's social security number for tax purposes. His mom & I are separated & I don't have access to his maternal files. How do I go about retaining his social security number?

Ask his mother. You say you need his SS number for tax purposes, but be aware that if the child does not live with you, you can not claim him as your dependent unless you get the custodial parent to sign off on form 8332 for you.