from the Last Will and Testament of John Poythress, 1712 (Prince George County wills) published in The Southside Virginian vol.IX no.4 page 153 transcribed by Virginia Lee Hutcheson Davis 
"In the Name of God Amen I John Poythres [Poythress] Senr. of Pr. Geo County .......... Item I give to my Son Josh Poythres three hundred Acres land at Monkanneck & to his heirs .......... Item I give to my Son Joshua Poythres two Negroes named Betty & Beck "
Joshua Poythress died in 1740 in Prince George County, his will written 17th January 1739/40, was probated on the 8th Apr 1740. and witnessed by his sister Christian 
A tentative list of the children of Joshua Poythress compiled by R. Bolling Batte in 1977
1. Joshua Poythress -1794 who married Mary Short (daughter of William and Mary Short of Surry County)
2. William Poythress 1753-1794 who married Mary Gilliam, daughter of John Gilham and Jane Henry of Prince George County.
3. Elizabeth Poythress who married Simon Fraser in 1775, in Middlesex County
4. Susan Ann Poythress 1766 - 1799 who married David Maitland in 1788 in Prince George County
Ancestors and descendants of Francis Epes I of Virginia: (Epes-Eppes-Epps) John Frederick Dorman; Society of the Descendants of Francis Epes I of Virginia. Publisher:[Petersburg, Va.] : Society of the Descendants of Francis Epes I of Virginia, 1992- Author: John Frederick Dorman; Society of the Descendants of Francis Epes I of Virginia, page 213
If born around 1689, Joshua is probably the son of John's second wife, not his first (Christon, bp 1634). See information and sources on this page on my speculation as to John's second wife.
R. Bolling Batte's research has Joshua as the first child (and John b c1665 as 6th). John (b c1665, m Mary Batte), was the son Christon (bp 1634). See RBB online chart and narrative. The narrative shows Joshua's son Joshua as husband of Mary Short (see info on first page of this JSTOR article).