When Simon McDermott’s father, Ted, was diagnosed with Alzheimer’s in 2013, everything changed. “It was a wake-up call,” Simon, 42, tells The Post. It made him realize that “Dad’s not going to be around forever.”

It was also the beginning of a dark time for the family. As Ted’s condition worsened, he became aggressive — a common side-effect of the degenerative brain disease. When he started dragging Simon’s mom, Linda, by the hair, Simon quit his marketing job in London and returned home to Blackburn, England, to protect his mom from Ted’s rages. Half the time, his dad didn’t recognize him.

But there was one memory not even Alzheimer’s could take from Ted: music. He had made a career for himself as a club singer, and while he never really made it big, he did well enough to travel around England as a resort singer, crooning classic hits. He earned the nickname “The Songaminute Man” from fans for his ability to sing any song perfectly on command.

Amazingly, Ted could still remember the words to those old songs. Simon noticed he even seemed happy when he was singing.

So Simon began creating “Carpool Karaoke”-style YouTube videos of them belting out standards such as “Beyond the Sea” and “Quando, Quando, Quando.” The series quickly went viral — their most popular video has more than 4.5 million views — and has raised more than £250,000 for Alzheimer’s Society. In November 2016, Simon and his father were awarded a Pride of Britain Award for their fund-raising efforts.

“It’s surreal,” says Simon, who’s now documented their incredible journey in “The Songaminute Man” — part memoir of Simon’s caretaking experience and part biography of his dad’s life, supplemented by interviews with his dad’s old friends and relatives. “My dad did have quite an unusual life,” he says. “I didn’t want that to be lost. I didn’t want him to be remembered [only] as a guy with dementia.”

While he says he “resents” the disease “and what it’s done to us,” he’s grateful that it’s afforded an opportunity for the two of them to spend time together.

“You could easily dwell on what I don’t have. But I try not to,” he says. “Even though dementia took my dad away, it brought us closer together and brought our relationship back.”