The charity’s invitation to take part in the network came as their Baton of Hope tour concluded in London on Friday, which Mr McCarthy described as a “fitting finale”.
Across 40 days, more than 1,300 people personally affected by suicide took turns to carry a specially made baton through 20 towns and cities across Great Britain and Northern Ireland to remember their loved ones and raise awareness of suicide prevention.
“It’s a great message of support and endorsement for them,” Mr McCarthy added.
In a letter penned to the charity, Prince William praised the Baton of Hope tour for its “vital message of hope and optimism across our nation”.
He wrote: “Together, you have united people and communities across the UK to open up a national conversation on suicide.”
Mr McCarthy said taking part in the tour had allowed him to feel closer to his late son.
“Ross asked me in his goodbye letter if I would fight for mental health support.
“I’m just so privileged and proud to be able to say to him ‘This is what you wanted’, and I’ll never stop.”