Owen Richards was 16 when his brother Joel Richards, 19, uncle Adrian Evans, 49, and grandfather Patrick Evans, 78, were killed in the Sousse beach massacre.
Prince Harry officially dedicated a memorial to the 30 British victims last year at Birmingham's Cannon Hill Park.
As the fifth anniversary of the atrocity approaches, Owen, now 21, from Wednesbury, has told how the Duke of Sussex gave him advice on how to deal with grief
He said: "It was a bit weird when he said, 'when my mum was killed', because she's not just any mum – she's Princess Diana.
"It seemed like he was trying to give some advice rather than have a meaningless conversation."
Owen also told how Harry gave him encouragement during his speech at the event.
"I was nervous but Harry nodded during my speech," he said.
"Mum and I had a teddy from our charity Smile for Joel – named after my brother – which we gave to him for his wife who was pregnant.
"I told him I was there when it happened and how I lost my brother, my grandad and my uncle."
Gunman Seifeddine Rezgui went on a murderous rampage on the beach at Sousse in June 2015 in what was the largest loss of British life to terrorism since the London 7/7 attacks.
The Smile for Joel charity, set up by Owen and his mother Suzy Evans, has raised more than £200,000 to support hundreds of victims of terrorism. For more information visit smileforjoel.com