The brave lad succumbed to an inoperable brain tumour on August 9... mum Elaine's 39th birthday. The 18-year-old had always lived life to the full, despite suffering from cerebral palsy.
But Elaine and dad Stuart were given the devastating news in April that Alastair had been diagnosed with a brain and spine tumour.
Stuart, from Balloch, Dunbartonshire, said: "The night before he passed, my wife whispered in his ear, he shut his eyes and went to sleep.
"It wasn't until he passed the next morning when I said 'what did you say?'
"She said, 'the best birthday present I could get is for you not to be in pain'.
"When my wife whispered in his ear, it was the first time he had shut his eyes in 72 hours and he never woke up again.
"I couldn't believe it when she told me what she said. It would bring a tear to your eye.
"When he took his last breath Shania Twain's God Bless The Child was playing.
"It has the lyric 'God bless the child that suffers', it was so appropriate."
Alastair's parents were told when he was just one that he would never walk.
But the determined youngster proved medics wrong and started walking when he was seven.
Proud Stuart - also dad to Andrew, 16, and Shannon, nine, - said: "Despite being told that he wouldn't walk or talk, Alastair excelled. From being in a wheelchair, he was walking at the age of seven.
"Alastair achieved everything he wanted to do in life.
"It was wonderful. He didn't let anything get in his way but when we were told in April that he had the tumours we couldn't believe it.
"The tumour was on his brain and spine but he then ended up with pneumonia.
"They said it was just bad luck with the tumour, then when he got pneumonia they said that was bad luck too.
"He must've been the unluckiest boy in the world."
Despite his illness, Alastair faced life with a smile.
Stuart, 40, added: "A few months ago he started falling over because the tumour was touching parts of his spinal cord, but we adjusted to it because he was still with us.
"It was hard for him because he knew what was going on around him. But he still had that infectious smile.
"He was a young man who loved cars, he always liked going out for a run in them.
"He met a number of famous people as well, including Billy Connolly and the guys from Still Game.
"He went from winning the best dancer award at his school Halloween party last year to being hoisted into his bed but he still had that infectious smile.
"We're still coming to terms with it all - it's still very difficult for us."
The heartbroken father revealed that he will always be grateful to the staff at Robin House, the children's hospice in their home town.
Big-hearted mourners at Alastair's funeral last Saturday raised £1,600 for the hospice.
His loved ones also had a red racing car made out of flowers for the moving service.
Stuart said: "The funeral was a celebration of his life. Alastair touched so many people's lives.
"They loved his infectious smile and his wicked sense of humour."
"Despite what he was going through he always had a smile."