I can find no evidence that C.H. Ross's uncle was Sir William Charles Ross. In fact I can find two reason, he is most likely not.
1) C.H. Ross is not listed in William Charles Ross's will - nor is C.H.'s father Charles. This is unusual in that William Charles Ross lists his father, brother John's widow (though doesn't give her a first name), Hugh, Magdalena and Maria. All of these people left no other heirs and while he supported them while they lived, once they died, the remainder of his estate was divided to his first and second cousins. I could list out these cousins but there were no Rosses included and except for the second cousins who I haven't identified yet, they are all related to William Charles Ross through his mother, Maria Smith.
2)William Charles Ross's father - William Ross died 7 June 1850 at Fitzroy-square - His obituary mentions his son Sir W.C. Ross and lists his age as 78. This was published in the London Daily News on 12 June 1850
3) C.H. Ross's father Charles can be found in the 1881 census living at 3 Oakfield St, in the civil parish of St Mary Abbott, City of Kensington, Parliamentary Borough of Chelsea. Charles is listed as 80 years old, born in St. Clements London and had the occupation of Director of the Times Parliam... (can't read the rest). He is listed as a widower and living with his sister, Georgina Ross age 78 who is also born in St Clements. This becomes useful later as I can find Georgina in the 1851 census
(Source: "England and Wales Census, 1881," database with images, FamilySearch (https://familysearch.org/ark:/61903/3:1:3Q9M-CS31-YSRJ-K?cc=2562194&wc=QD53-RZ9%3A1588889043 : 20 August 2016), 101774323 > image 1146 of 1220; from "1881 England, Scotland and Wales Census," database and images, FindMyPast (http://www.findmypast.com : n.d.); citing PRO RG 11, The National Archives, Kew, Surrey.)
4) Georgina and Thomasina Ross are listed in the 1851 census with their father, William Ross. If the Georgina listed in the census living with her brother Charles in 1881, then this would be the same Georgina. Since W.C. Ross's father died in 1850, there is no way that this William Ross could be the father to Georgina and Thomasina (and by inference Charles RossJ) Source Citation Class: HO107; Piece: 1512; Folio: 83; Page: 17; GSU roll: 87846
Source Information Ancestry.com. 1851 England Census [database on-line]. Provo, UT, USA: Ancestry.com Operations Inc, 2005.
When you combine these pieces of information. Claiming William Charles Ross as his uncle can not be supported by the evidence and is most likely a brag. While these two William Ross might possible be cousins, I don't see any evidence to support this.