Elliot Eland was born with Angelman syndrome - a chromosome disorder which causes severe learning difficulties and a permanent smile on his face, the Mirror reported.
Sufferers are often smiling or laughing, and tend to be easily excited.
The condition is so rare that less than 1,000 cases have been reported in the UK.
Eland’s mum, Gale, said: “When we feel down, Elliot’s laughter keeps us all going, you just have to look at him and his happiness takes over.
“At the end of the day when you have children you just want them to be happy, and Elliot always is.”
“The only problem is if Elliot and his big brother, Alex, are playing and Alex accidently gets hurt. Alex will be crying, but Elliot just laughs and he doesn’t understand why. That can hurt as we don’t want Alex to think Elliot is happy about hurting him,” she said.
Gale, 41, and husband Craig, 34, only discovered Eland’s condition when he was having trouble feeding as a baby.
“I didn’t think Elliot had a major issue, he just seemed to have feeding issues - even the doctors and midwives didn’t seem too concerned to begin with,” Gale said.
“On his six week health check the doctor referred him to Royal Preston Hospital, where he was seen by a consultant paediatrican who admitted him immediately.
“During his time in hospital he had countless blood tests, MRI scans and x-rays. His paediatrician luckily had seen Angelman’s Syndrome once before and had his blood tested for it. It was only when we were told that he needed to have these genetic tests that I became wary. It was a total shock when the tests came back positive and we were told that he had Angelman’s - I’d never heard of it before,” she said.
Gale added : “We were driving home in the car and my husband was googling Angelman’s.
“He started reading all these things off the computer and in the end I was sobbing so much I had to pull over the car.
“It was so hard to begin with because we knew so little about it. It was only thanks to charities and support groups that we were able to cope,” she said.
Gale and Craig see Eland’s smiling as a happy side effect of an otherwise difficult condition.