Not-so-little Archie Thompson's bulk could kill him. He is not yet two years old, yet he tips the scales at six stone - three times the normal weight for a boy his age.

Archie, from Icklesham, near Rye, is the only child in the UK with a rare genetic disease known as Momo, or macrosomia obesity macrophaly ocular abnormality syndrome.

With his unfortunate obesity come heart and breathing problems, poor eyesight and learning difficulties.

Yet despite his catalogue of health problems, Archie was only diagnosed four weeks ago, so rare is the condition.

He was born 23 months ago, weighing a healthy 8lb 4oz.

He was slightly heavier than his two elder sisters had been at birth but his parents, Sarah and Nigel Thompson, put this down to his gender.

Mrs Thompson said: "The other two were only 6lb but I thought as he was a boy it was fine. At six weeks he doubled in weight and he just kept piling it on."

With no history of health problems and two healthy girls, Mr and Mrs Thompson could only watch with concern as their son's health continued to decline.

While the full implications of Archie's condition are still sinking in, Mrs Thompson is pleased she can finally put a name to her son's symptoms.

She said: "I suppose it was a relief that we got a diagnosis but when I realised he was the only one in the UK and there was no cure, it didn't make an awful lot of difference.

"They can't tell us how long his life will be. The doctors have said to us the best way for us to live is day by day. I have only just started planning Christmas this week."

Archie's doctors were only able to fully understand his condition after liaising with colleagues overseas and scrutinising the results of an eye test.

Only four children in the world are believed to have Momo syndrome, one in Italy and two in Brazil.

No one can say where the disease comes from, only that it is a rare genetic syndrome.

Dr Fred Kavalier, of the genetics department at Guys Hospital in London, said: "Very little is known about Momo syndrome.

"It is very rare and patients can suffer from a number of similar features such as developing large heads, obesity and eye abnormalities."

Archie is expected to continue to gain weight, at about one pound a week, placing increasing pressure on his heart and lungs.

It is hoped to get an intravenous line inserted into his chest so nurses can feed vital drugs into his veins quickly when he becomes ill.

The need for the line became apparent last summer when an abnormal heart rhythm developed into a heart attack.

Yesterday, the family was hoping Archie would be allowed home from the Conquest Hospital, Hastings, after an asthma attack landed him back on a ward.

Mrs Thompson said: "I wouldn't like to say how many times we have been back and forth to the hospital. If we're not here, we're in hospital. They are the only two places he goes."

Despite the need for hospital intervention, doctors agree the best place for Archie is at home to reduce the risk of picking up further germs and illnesses.

Looking after three children is a full-time occupation for 34-year-old Mrs Thompson and Archie alone needs 24-hour care.

His breathing problems and obesity mean he has to sit propped up to sleep, under sedation, and often needs to be moved to stop him getting cramp.

Archie's bedroom doubles as a playroom and eating area.

His size makes it difficult for Mrs Thompson to move him around and getting out of the house is almost impossible.

Ordinary car seats simply will not take his weight and booster seats for older children will not restrain him.

Four-year-old sister Mollie pines during his frequent spells in hospital, while 11-year-old Robyn has been teased at school because of her brother's size.

Mrs Thompson fears the two girls have missed out on a lot of her attention because of the devotion Archie requires.

Next year he is due to start at Glyne Gap, Bexhill, a school for children with severe learning difficulties. He will receive physiotherapy, hydrotherapy and sensory stimulation.

A number of days have also been allocated for Archie at Demelza House, a children's hospice, which offers respite care for families of seriously ill children.

Getting Archie to and from Bexhill and the hospice in Sittingbourne will nevertheless be tricky.

The family has no means of transporting their son or his wheelchair and lifting Archie into the family car has proved too much for his mother.

Although registered as disabled, his age means the Thompsons are not eligible to apply for a specially-adapted car, which would solve their problems.

Sadly Archie may not live long enough to get such a vehicle from the local authorities.

Mrs Thompson said: "The authorities have never come across a case like this before.

"Doesn't he deserve to get out with the family to see the Christmas lights and see all the things in the town that are pretty?

"He might not get a chance next year."

So determined is Mrs Thompson to get her son out of the house, she has written to the Prime Minister.

In a letter to Tony Blair, she said: "I find it ironic Archie is entitled to a blue badge because he is disabled but has to wait maybe one or two years for help towards a mobility vehicle, which I am sorry to say Archie needs now.

"If we have to wait we may not have Archie.

"Through all the stress and the fighting, I just desperately want Archie and my family to be happy. We are just a small family with a special little boy.

"I am fund-raising around the clock to try to raise enough funds for a second-hand vehicle, which costs in the region of £8,000.

"My son's condition is life-limiting and life-threatening. I am just a normal mum who wants to make his quality of life as it should be for a growing child - as I'm sure you do your children."

The Thompsons are determined to get the vehicle they need to take Archie on family outings with his sisters - while they can.

To help the family, call Mrs Thompson on 01424 814934.