Peter Capaldi’s Doctor Who may be one of the more sinister incarnations of the much-loved TV character British television has seen.

But the actor never-the-less showed his softer side when he sent this heart-warming YouTube message to a nine-year-old boy diagnosed with autism – a lifelong developmental disability that affects how a person communicates to, and relates with, the people and environment around them.

Thomas had apparently sent a letter The Doctor as he was preparing for the funeral of his grandmother.

The contents of the letter are not known, but receiving the clip from Capaldi, his mother said, helped him to cope with the grief he was newly feeling for the first time "in a profound way".

"Sometimes sad things happen to us," Capaldi says in the video, "So it’s nice to know that here’s somebody out there who’s on our side..."

"Wow. Had no idea this would go viral. We only posted it so family could see it after the funeral," Ross Goodall, Thomas's father, has since posted in the comments section of the YouTube clip.

"Made such a huge difference and helped him cope with the hardest of days. It was an incredibly kind thing to do that has help Thomas gain perspective and not feel so alone in his grief. Where Thomas is autistic, he doesn't feel emotions the same way the rest of us do so this has been incredibly hard for him. This message seems to have given him the strength to keep going, with his chin up."

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"Wish I could thank Mr Capaldi in person," he added.