Luis Manuel, who is just 10 months old, weighs in at 30 kilogrammes (four stone 10 lbs).
Doctors have told his parents, from the city of Tecoman in the western Mexican state of Colima, that they fear the youngster may have Prader-Willi syndrome (PWS).
His family have launched a fundraising appeal as they say they will not be able to afford medical treatment for their little boy if the diagnosis is confirmed. Treatment would consist of a series of hormone injections, with each one reportedly costing 10,000 MXN (about £404).
Luis Manuel weighed a healthy 3.5 kilogrammes (7 pounds 11 ounces) at birth but began to put on weight at an alarming rate. His mother, Isabel Pantoja, said: ‘After only one month we noticed that clothes did not fit him and we had to dress him in clothes for a one-year-old, and even a two-year-old.
‘We saw our baby gain weight so fast. Sometimes, he could not sleep because he felt like he was suffocating due to his weight.’
Doctors say Luis Manuel is currently the normal weight for a nine-year-old boy even though he is still two months short of his first birthday.
Prader-Willi syndrome is a genetic disorder caused by a loss of function of specific genes which causes sufferers to feel constantly hungry, often leading to obesity and type 2 diabetes.
Doctors in Tecoman say Luis Manuel would be the first ever recorded case of the condition in the city.
Mario Gonzalez, the boy’s father, said he was worried about the health implications for his son. He said: ‘In some cases, kids have died because of a heart attack due to being so overweight.’