Benjamin son of William was still with family and unmarried in 1851. He can therefore be eliminated. This is the Census entry:
Piece Folio SchNo Town Surname Forename Relatn Marr Age Occupation County Place
1959 0166 151 Drybrook TRIGG William HD W 48 Cordwainer GLS Dean Forest
1959 0166 151 Drybrook TRIGG Ann WI M 44 - GLS Dean Forest
1959 0166 151 Drybrook TRIGG Benjamin SO U 21 Cordwainer GLS Abinghall [Abenhall]
1959 0166 151 Drybrook TRIGG William SO U 19 Coalminer GLS Abinghall [Abenhall]
1959 0166 151 Drybrook TRIGG Joseph SO U 15 Coalminer GLS Abinghall [Abenhall]
1959 0166 151 Drybrook TRIGG Maria DA U 13 At Home GLS Abinghall [Abenhall]
1959 0166 151 Drybrook TRIGG Sarah Jane DA U 11 At Home GLS Ruardean
1959 0166 151 Drybrook TRIGG Absalom SO U 9 At Home GLS Ruardean
1959 0166 151 Drybrook TRIGG Eliza DA U 7 At Home GLS East Dean
1959 0166 151 Drybrook TRIGG George SO U 4 At Home GLS East Dean
1959 0166 151 Drybrook TRIGG Priscilla DA U 2 At Home GLS East Dean
1959 0166 151 Drybrook TRIGG Mary DA U 5M At Home GLS East Dean

Mike

HO 107 /365 / 10 folio 24 Ruardean
Morse
William TRIGG 38 Corwainer (sic)
Ann 35
Benjamin 11 ( could be 17 )
William 16
Emma 7
Joseph 4
Maria 3
Sarah Jane 1
Absolom 5 wks
All born in county

HO 107 /364 / 7 folio 4 (?) page 6 Dean ex Parochial
Haseley / Drybrook
Benjamin TRIGG 18 Cole miner (sic)
Edwin 11
Absolom 8
All born in county

From GFHS Marriage Index:
20 September 1813 Benjamin TRIGG, Extra Parochial, married Elizabeth ROBERTS, Extra Parochial, by Banns at English Bicknor.

From GFHS Burial Index:
Date: 12 JUL 1836
First Name: BENJAMIN
Surname: TRIGG
Age: 47
Burial Register: EAST DEAN(DRYBROOK) HOLY TRINITY 1817-1839
Place of Abode: DRYBROOK

GFHS Marriage Index:
14 January 1837 John GLOVER married Elizabeth TRIGG at Longhope.

1851 Census:
Piece Folio SchNo Town Surname Forename Relatn Marr Age Occupation County Place
1959 0164 137 Deans Meand TRIGG Benjamin HD M 26 Coalminer GLS Ruardean
1959 0164 137 Deans Meand TRIGG Mary WI M 21 - GLS Newland

1959 0164 139 Deans Meand GLOVER John HD M 45 Coalminer GLS Westbury on Severn
1959 0164 139 Deans Meand GLOVER Elizabeth WI M 55 - GLS Dean Forest
1959 0164 139 Deans Meand TRIGG Edwin SS U 22 Coalminer GLS Ruardean
1959 0164 139 Deans Meand TRIGG Absalom SS U 18 Coalminer GLS Ruardean
1959 0164 139 Deans Meand GLOVER George SO U 11 Scholar GLS Ruardean
1959 0164 139 Deans Meand POWELL William LG U 22 Iron Miner GLS Mitcheldean
1959 0164 139 Deans Meand ROBERTS Elizabeth SV U 12 General Servant GLS Forest of Dean

Baptisms at Drybrook Chapel:
14 August 1851 Leah TRIGG. Parents Benjamin TRIGG, Collier East Dean and Mary
2 April 1854 Alice TRIGG. Parents Benjamin TRIGG, Collier East Dean and Mary
6 September 1855 Mary Jane TRIGG. Parents Benjamin TRIGG, Collier East Dean and Mary

Mike

If so I'll see what I can find in 1841 census - hopefully he shows as 15 and living at home ....