The document, which doesn't include an estimate of Brown's estate, begins, "I, James Brown, also known as 'The Godfather of Soul' ... have six living children."
The bequest was signed 10 months before the child's birth and more than a year before Brown was married to Hynie.
Attorneys who handled the will told the AP that the child may be entitled to some of the estate, but a paternity test would be needed.
"We would not want to divide and deliver property until all these issues are resolved," said attorney Ray Massey.
The will says Brown intentionally failed to provide for an "any other relatives or persons whether claiming, or to claim, to be an heir of mine or not."
Brown died of heart failure in an Atlanta hospital Christmas morning at age 73.
Most of Brown's "primary assets," including the singer's music rights and his 60-acre Beech Island estate, are in an irrevocable trust separate from his will, according to Brown's attorney and trustee Buddy Dallas.
Dallas didn't know how much the trust was worth.
On Thursday, Brown's body remained in a sealed, gold casket in a temperature-controlled room at his Beech Island home, said Charles Reid, manager of the C.A. Reid Funeral Home in Augusta, Ga., which handled Brown's funeral.
His children have said through an attorney they are planning to turn his home into a Graceland-like museum and build a mausoleum on the site for his body.
The children named in the will are Deanna J. Brown Thomas, Yamma N. Brown, Vanisha Brown, Daryl J. Brown, Larry Brown and Terry Brown.
The will, and Hynie and her son's exclusion, has added to a dispute about Brown's legacy.
Brown's attorneys contend that Hynie is not Brown's widow because she was still married to another man when they said their vows. They have said Hynie later annulled her previous marriage, but she and Brown never remarried.
"That is going to be litigated and a judge will tell us whether she is Mr. Brown's spouse," attorney Strom Thurmond Jr. said.
If Hynie was an omitted spouse, she could receive a third of Brown's estate, Thurmond said.
Referring to whether James Jr. would be part of the will, Thurmond said, "there's a cloud over this issue as well."
Hynie's attorney, Robert Rosen, said Wednesday that he will sue, asking a court to rule Hynie is legally married to Brown and is entitled to half of his estate.
"At the end of the day, in my opinion, Tomi Rae Hynie will inherit 50 percent of James Brown's estate and the trust and the child, James Brown's son, will inherit one seventh of the other 50 percent," Rosen said.
Phone messages left for Hynie and Rosen were not immediately returned Thursday.
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