The Duke of Sussex and former Wales rugby star Gareth Thomas joined forces to highlight the importance of getting checked out ahead of National HIV Testing Week, which will run from November 16 to 22.
The pair met players from the London-based Kings Cross Steelers - who style themselves as the world's first gay rugby club - and members from Premiership rugby club Harlequins.
They gifted Harry with a rugby kit for his son inscribed with the little boy's name.
Mr Thomas praised the Duke of Sussex for his efforts in raising awareness about HIV testing at the event.
The welsh champion, who revealed in September that he is HIV positive, said Harry has "done so much to normalise HIV testing and fight the stigma across the globe".
The former player's comments came as he was named as a member of the Terrence Higgins Trust's new HIV Commission.
Thomas said of his appointment: "I spoke out about living with HIV, not for me but for all those people who are struggling and don't have a platform.
"For them, I want to do everything I can to challenge stigma and outdated views about HIV.
"That's why I'm thrilled to be joining the HIV Commission because I want to be a part of a positive change and play a role in driving us towards our goal where no-one else contracts HIV."
Thomas was the highest-profile sportsman in the UK to reveal he was gay when he came out in 2009 and earlier this year said he was forced to reveal his HIV diagnosis after a tabloid newspaper threatened to publish it.
The ex-fullback, who captained both Wales and the British Lions, is thought to be the first UK sportsman to go public about living with HIV.