He had son David John c 1788 - 1867 possibly married Elizabeth.
This David had a son David born Pembrook 1820
I know the name John may not be the real original surname,
Not quite sure what you mean by “John may not be the real original surname” as this surname is long and well established through many centuries as I know from my own research.
From my transcription of the Parish Registers of Pembroke St Mary 1711 to the 1950’s I note in the baptism register for the period 1813 to 1822 the following ;
25 May 1813 John to John & Elizabeth John of Pembroke a Farmer by Thos Dalton
27 Apr 1814 Jean to John & Elizabeth John of Pembroke A Farmer by Thomas Dalton Twin Daughter
27 Apr 1814 Elizabeth to John & Elizabeth John of Pembroke a Farmer by Thomas Dalton Twin Daughter
21 May 1815 John to John & Elizabeth John of Pembroke a Butcher by Wm Lloyd
28 Aug 1816 Anne to John & Elizabeth John of the Green Pembroke a Butcher by Wm Lloyd
02 Nov 1817 Anne to John & Elizabeth John of Imble a Farmer by Wm Lloyd
07 Feb 1819 Mary Anne to John & Elizabeth John of Imble a Farmer by C. Philipps
10 Feb 1820 David to John & Elizabeth John of Imble a Farmer by C. Philipps
10 Feb 1822 Mary to John & Elizabeth John of Imble a Farmer by C. Philipps
There are no other references in the area of your interest to any other baptism of a David John circa 1820.
Imble at this time was in a very rural area known as Paterchurch and part of the parish of Pembroke St Mary. 1814 saw the start of the brand new laid out town called Paterchurch Dockyard. Paterchurch Dockyard would change is name and thus the town of ‘Pembroke Dock’, was born in July 1817.
Part of Imble Farm still remains although most is a housing estate. Imble Lane leading to Imble Farm was the scene of 3 tragic deaths during W.W. 2.
The parish church of Pembroke Dock would be Pembroke Dock St John and start services in circa 1844. Although the area would remain under Pembroke St Mary and Pembroke Borough till well into the 1860’s, although by this time P.D. Elders where flexing their muscle querying why they where paying more for gas street lighting with fewer lamps that Pembroke Town was with more lamps.
The only mrge of a David John I can establish in one of the many parishes covered by the Pembroke Registration District took place in the 2nd Qtr of 1849 and I can confirm did not take place at Pembroke St Mary or Pembroke St Michael, by 1849 it may well take place at Pembroke Dock St John.
Rgds
Orielbenfro
ORIEL a welsh window on a surname