The genetic disorder, which affects one in 30,000 people worldwide, causes a wide range of physical symptoms, learning disabilities and behavioural challenges.
Typical signs include restricted growth, floppiness and an excessive appetite, which can easily lead to dangerous weight gain.
Despite Mark's five daily meals being served on time - breakfast at 7am, morning snack at 11am, lunch at 1pm on the dot, an afternoon snack at 4pm, and dinner at 7pm - the young lad is hungry 24/7.
His parents, Aga Činari, and lawyer Marc Allepuz-Rico, 41, have accepted they will never be able to fulfil his hunger and the only thing stopping him from becoming obese is their dedication to a carefully controlled diet.
Aga, 46, from Aberdeenshire, said: "He completely obsesses about food. When I found out the news, I spent every waking minute googling everything about PWS and preparing for the worst because I had never heard of it before.
"It was the scariest thing ever.
"The time I should've spent enjoying my baby and making memories was spent going to different doctors and it never seemed to end."
Aga cares for Mark full-time and was "extremely excited" when she found out she was pregnant in February 2015. During the pregnancy, she had "little suspicion" anything was wrong but would notice he didn't kick much and there was little movement.
In August, Aga had a private ultrasound scan and was told her baby "looked fine". On November 19, 2015, Mark was born via C-section - two weeks late - and she immediately noticed he wasn't making any noise and "felt extremely floppy".
"None of the doctors knew what was going on," Aga said.
"At first, they thought he might have had an infection and he was taken to ward after ward. He wasn't feeding well and wasn't crying"
A genetician suspected he might have PWS, which was confirmed three weeks later after tests.
"The doctors said he would struggle to walk and talk and that he would be delayed in his g...