The boy's results in nine GCSE exams - four A- grades, three Bs and two Cs - are "not going to set the world alight," his father said yesterday, but he hopes they will be the key to getting Rhys reinstated at the £21,900-a-year Marlborough College in Wiltshire.
At home in his Knightsbridge mews house yards from Hyde Park, one of two properties he owns in a street where homes change hands for £1.3m, Mr Gray, 49, says he is more confident about the battle ahead.
The property restorer, who has had custody of Rhys and his brother Harley, 19, since Rhys was a toddler after separating from their mother, is risking more than £100,000 in legal bills in an attempt to force Marlborough to accept Rhys.
"You would have to be pretty foolish to launch a litigation that you cannot afford to see through," he said. "If you embark on litigation then you had better budget for it. I am confident that we have a good case but it is always unpredictable."
The school has said the expulsion is for Rhys's "behaviour and attitudes", and the teenager admits he has accumulated about 400 "chits" at school for minor offences.
Mr Gray dismisses suggestions that his battle is a rich man's whim. "If I feel I have been badly treated or let down by somebody who has behaved dishonestly I will look for reparation," he said. "Most people I know think I'm a bit obsessive, in that I would invest disproportionate effort in righting a wrong. They think just let it go, but if I think I have been stitched up or taken advantage of, then if there's a way of achieving reparation, then I will work as hard as I can to achieve it. So I'm sure there's a measure of that in this.
"But [this is] more about how to achieve the best outcome for Rhys. You can't confront someone at the last minute with the need to find an alternative school place. It isn't that easy and I am convinced Marlborough is the best place for Rhys." He admits he would have abandoned his battle if Rhys's results had not been "significantly better" than the three Bs and three Cs that Marlborough has set as the minimum standard for entry into its sixth form.
For Mr Gray, his son's A-grades in all the subjects he intends to study at A-level - maths, physics and chemistry - are a vindication of his son, and undermine the school's case that Rhys is "unwilling or unable to profit from the educational opportunities on offer".
In a 50-page school statement detailing Rhys's behaviour, Nicholas Sampson, the master, said Mr Gray had unrealistic expectations of his son's academic capabilities. Mr Gray's belief that Rhys would get A-grades in his A-level subjects was "unrealistic". He was more a B or C grade, he said.
At home, Rhys does not display the arrogance expected of a boy seen as a tearaway. He said he was surprised at the portrayal of his father by some as a lax parent who has conspired to undermine school discipline by condoning bad behaviour. "He's much stricter than any of my friends' parents," he said.
Mr Gray, who also has two children, aged eight and six, with a new partner, said: "I have been as rigorous and firm in my disciplining of Rhys as you can be, and I am well aware that he is no angel."
But there are parallels with Mr Gray's upbringing. When he was 11, his state primary school told his father he was not of sufficient calibre to benefit from a grammar school. Although his parents were not rich, his father, an importer, sent him to a private grammar school in Twickenham, then to the London School of Economics to read philosophy, then a Masters degree in economics.
"My background was to have been underestimated and written off by one institution or another," Mr Gray said. "My inclination is to view them with suspicion. I think that's what they cannot handle, someone who questions what the school says."
But if Rhys is allowed to return, will his father insist he behaves? "I have already made it clear to the headmaster that I would be expecting higher standards of commitment and maturity from Rhys in the sixth form than even the school does. Rhys is well aware I have high expectations. I do not dispute the right of independent schools to choose who they educate. But when they select someone who turns out to be more of a challenge than expected, they shouldn't be allowed to drop him like a hot brick unless he has done something deserving of expulsion."Reuse content