When a child is entitled to his stepfather's property

Dear Harold,

I have been married for 18 years and have four children (three sons and a daughter). During marriage, I had an affair with a married man who fathered my lastborn son who is nine years old. My predicament is my husband knows that the child is not his and has threatened to exclude him from his estate (inheritance). However, he treats the child just like the other three that he biologically fathered. Does the child have legal rights over the property of my husband? Would I have any legal options should my husband make good his threats of excluding the child from his will.