Chris Packham broke down in tears when he read a letter from Ken Bruce's autistic son, Murray.
The Springwatch star, 61, met up with the former Radio 2 broadcaster, 72 and Murray, 20 - who has never spoken and communicates via a tablet - on his BBC show Inside Our Autistic Minds.
After meeting the father and son, Chris, who has Asperger's Syndrome, asked Murray to come up with script ideas for his own film.
While filming for Springwatch, the presenter took a moment to read his emails and was in tears when he saw Murray's.
Chris began reading: "So I asked if you had all the world stage to speak for non-speaking autistic people, what would you say?
"He's really listened to me. Take note as I am really speaking from my heart, I need you to please understand and open your minds."
The presenter paused for a moment before he continued: "Wow."
The email went on: "Let me tell you what it was like for me before I could communicate through typing.
"I was alone in my world of deep and troubled pining for a person to hear me. I would wonder if that day would ever come.
"I was frustrated and pining for acknowledgement that I had thoughts and opinions and worries and no one could hear me."
Wiping his eyes, Chris said: "It's enough to bring a tear to your eye, isn't it?
"I don't know whether it's some form of affinity for Murray but I find it incredibly touching and a bit sentimental.
"I am not usually sentimental about such things."
Viewers praised the naturalist for the programme, with one saying: "Just wow for Chris Packham's - Inside Our Autistic Minds - had us in tears, had us helping us to understand our son better and had us talking about who he is."
While another said: "Chris Packham's BBC2 programme on being autistic was just superb. I sobbed all the way through. So uplifting and stupendous. Do see."
Chris was diagnosed with autism in later life, but said his childhood was scarred with bullying.
Speaking on the show, he said: “I have a visual memory of some of the most horrific things that other young people said to me when I was a kid and they still hurt now.
“They’ve tattooed something unpleasant in my psyche.
“I am so pleased a particular child in my early life could never get to me in my bedroom. He made my life absolute hell."
