What a pity if he had only the one child Robert George Christian whose birth cert gives only Isle of Man as his father's birth place. I was hoping that there might have been other children whose birth certs were more detailed.
My theory last year was based on what you wrote about Robert Christian's death cert., which had his father as Robert Christian and his mother as Margaret Anne Christian formerly Boyd, farmer.
We thought he might have been illegitimate, and the only Robert Christian b.c.1847 with a Margaret Boyd connection who I could find was in the 1851 census of German, IOM, at Ballacrop.
The Head of Household was Catharine Boyd, unmarried, aged 60, with Margaret Barr, sister, widow, aged 55, Wm Wade son-in-law mar., 24, farm labourer, Margt Wade, sister's da., mar., 27, milinor, and three nephews: Wm Currin 9, Robt Christian 4, Wm Wade 1, all born German.
The nephews all appear to be the children of Margaret Wade als Barr aged 27. My theory was that Robert might have thought that his mother's maiden name was Boyd, not Barr, because that was his grandmother's maiden name and they were all living with gt aunt Catherine Boyd.
The IGI has a bapt for Robert Barr 14 March 1847 Kirk German, mother Margt Barr. No father given (illegitimate). If his father was Robert Christian he would be known as Robert Christian in the Manx way, even if his parents never married.
However when checking the 1851 census entry again, I found another possibility, also in German, at Close E Gary. In the 1851 census there is a Robert Christian aged 3, the son of William Christian aged 29, Farm Labourer, b.Maughold, and his wife Ann aged 28 b.Michael. Also children William aged 5 and Ann aged 1 b.German, AND mother-in-law Ann Boyde Widow, 55, Herbalist, b.Michael.
You have his mother as Margaret Anne Christian formerly Boyd so I thought she could have been known as Ann, but unfortunately she appears on the IGI only as Ann (a member's entry, replacing whatever was there before from the parish register).
So the first census entry above is (so-far) still the better choice to have been your Robert.
Sue