Agnes
Sandwell council didn't exist in 1908. It was formed only in 1974. Today Sandwell is made up of Oldbury, Rowley Regis, Smethwick, Tipton, Wednesbury and West Bromwich. In 1908 all these towns were in Staffordshire or possibly Worcestershire. Have you contacted Staffordshire Record Office or Lichfield Record Office?
http://www.staffordshire.gov.uk/leisure/archives/In 1908 there was no council social services only poor law unions. At least some of the towns now in Sandwell were in Dudley poor law union.
http://users.ox.ac.uk/~peter/workhouse/Dudley/Dudley.shtmlWorkhouse records held by Dudley archives and possibly any other poor law records (this concerns children who came into poor law care) for those towns in Dudley union.
http://www.dudley.gov.uk/leisure-and-culture/local-history--heritage/archive-and-local-historyThe other towns were in West Bromwich poor law union
http://users.ox.ac.uk/~peter/workhouse/WestBromwich/WestBromwich.shtmlAny surviving poor law records are probably with Staffordshire archives.
The poor law unions were the equivalent of council care today. Their records are likely to have been handed over to record offices before Sandwell council came into existence. I doubt people working for Sandwell social services today have much historically knowledge of poor law unions, which legally ceased to exist in 1930, but in reality finally gave up the remainder of their power with the formation of the Welfare State in 1947.
I can't comment on the 1871 census child because all the information you give is for a foster child only somewhere in Staffordshire. I checked the 1871 census index for foster children in Staffordshire and none came up living with a family called Guest, so I don't I'm afraid know which area of Staffordshire he was living in or where he was born - necessary knowledge for where records concerning him might be held today.
Your figures don't seem to add up - the foster son died aged 20 in 1908 but was on the 1871 census?
Regards
Valda