If he eats food with carotene in, it builds up in his body and turns him orange.
His mum, Angie, 40, has been forced to give Leo a strict diet, eliminating all produce with carotene in.
Medics took months to discover the root of Leo's condition when his worried mum first raised concerns over his orange skin when he was just six months old.
Doctors at Basingstoke and North Hampshire Hospital had never seen a case like it. His mum thought the orange tinge was connected to the pneumonia he had suffered as a baby:
She said: "He'd had pneumonia and we were very close to losing him. When he was ill, his body started to shut down. His kidneys and liver had been failing so that's where we thought the orangey yellow colour had come from to start off with. We just assumed that was what the problem was and it would go, but a month later he was still orange."
Specialists initially said Angie was 'feeding him too many carrots' and was ordered to cut out all orange foods from his diet. Angie kept a food diary for three weeks to prove to the docs she was NOT giving him orange food, but Leo remained the same colour.
She told the Daily Mail: "He was a very quiet baby. At six months, they are usually trying to crawl but he was not wanting to do anything. It was another reason that made me think someone else was going on."
Eventually blood tests showed Leo was missing a liver enzyme preventing his body from converting carotene into Vitamin A.
Angie, who has three older children, Robert, 19, Peter, 16, and 11-year-old Melissa, said strangers used to stop her in the street complimenting her on Leo's 'tan':
"I'd have to explain he had a health condition that turns him that colour."
She says she is relieved to have got a diagnosis for Leo but would like to raise more awareness of the condition.
"As far as I know, he's the only one in the UK with it. It's all new to them at the hospital," she said, "The doctors haven't said what the carotene could do. The only way to find out is to feed him lots of carrots but I wouldn't want to do that. I know the carotene would probably make him ill because his body has got no way of getting rid of it.
All the stuff that's really good for you, he can't eat. The only vegetable he can eat is cauliflower.
Despite his condition, Angie says Leo is just like any other youngster, and is 'on the go' all day long.
He is such a happy little chap. We used to call him the Tango kid and Oompa Loompa because of his colour. He looks like someone from The Only Way Is Essex when they have overdone it on the fake tan.