Lewis, aged 13, is living on borrowed time because he suffers from a condition which means doctors did not expect him to live beyond the age of 12.
But his family are determined he will have a star role when dad James marries his fiance Emma.
James and ex wife Samantha Tolmie split in 2008, when Lewis, who suffers from a recently discovered variant of the terminal illness Batten Disease, was four.
But the couple remained friends and Samantha regards Emma and her daughter as part of the family.
When Lewis was first diagnosed, aged two, the doctors told his parents he would probably only live until he was 12.
He has already confounded the experts by reaching 13, so when they decided to marry, James and Emma wanted Lewis involved.
The legal ceremony is in Portugal in 2018.
But James and Emma are holding ‘wedding part one’ on September 2, with Lewis as best man, at Samantha and Lewis’ home in Bessacarr.
It will not be a legally recognised but will include every element of a wedding, including over 100 guests.
James said Lewis developed normally until he was two, walking and talking, greeting everyone with a big ‘hiya’ and chasing the cat around the house.
Then one day he began limping. He lost the use of his legs, then his arms, then he couldn’t swallow.
He is on a ventilator and is prone to epilepsy and respiratory disease. He can no longer walk, move, or speak.
But Samantha says he can communicate with those who love and care for him using his eyes.
Samantha said: “James and Emma really want Lewis to be there but he is not going to be able to be there next year - that is why they’re having wedding part one.
“Emma will be in her wedding dress. Her six-year-old daughter Poppy will be in a bridesmaid dress, and Lewis will be the best man.
“He is too ill to be able to talk about it, but he adores everyone, and he will really enjoy this.
“You can’t help but love Lewis. He’s adorable. He is cheeky and has the most wonderful smile.
She added: “We’re so happy that we can do this for him.”