Jason Freeman claims he's the son of Charles Manson, Jr., which therefore makes him the grandson of the notorious head of the Manson Family. He's gunning to become the administrator of the estate and the sole beneficiary of Manson's estate.

Here's the problem for Freeman ... Manson left a will, leaving all of his assets, including his "exclusive music catalog" to one of his pen pals, named Michael Channels.

Freeman, who claims Manson was worth millions at the time of his death, wants to invalidate the will and step in position to inherit the estate as next of kin. Freeman's dad died in 1993.

The Estate has asked Freeman to take a DNA test to prove one way or the other if he's related, but according to legal docs, he's repeatedly refused. So, the Estate is now asking a judge to order a DNA test.

The Estate concedes this ... an Ohio court found Freeman is related to Manson, but the estate calls the ruling "muddled."

As for the pen pal ... he and Manson exchanged letters and phone calls for 2 decades. The pen pal occasionally visited Manson in prison. Manson's handwritten will looks like the real deal, but Freeman clearly is not convinced.

As for what Manson really has ... he did write a Beach Boys song that made the B side of one of the group's singles back in the day.