He must be connected to the family of Sir Russell James Kerr - the Andrew Kerr, nephew, was born 20/5/1906 (had a female twin), son of Sir R's brother Mark Kerr.
In 1947 said Andrew arrived from South Africa with wife Ismay Mildred, plus a 6year old son (also shown in Gloucestershire electoral rolls at same time as Roy G.) and 2 year old daughter so suspect Roy G is a son of Andrew and Ismay but the couple don't seem to have married in the UK, nor the 2 known children born in UK, probably South Africa but whether Roy G. was born before possible 2 siblings I can't say.
The men in the Kerr family all seemed to serve in the army so could be how Andrew ended up in South Africa. The known son bc.1940/41 was still in Newnham, Goucestershire on electoral rolls so still alive in 2019. If Roy G. not a brother, perhaps an uncle (brother of his father Andrew) but can find nothing to substantiate this.
Roy G. seems somewhat of an enigma at the moment but must fit into this family somewhere.
Annette