Six-year-old Sam Brown, of Otley, near Leeds, is one of only 78 children in England with Morquio syndrome.
The condition prevents youngsters growing properly, forcing their organs into an ever-smaller space, leading to severe physical disability and triggering a range of other problems including hearing, sight and heart disorders.
The condition has until now been untreatable but Sam is involved in a trial in Manchester of a drug called Vimizim which has shown success in slowing its progression, giving him a better quality of life.
NHS England was forced to back down last year after a legal challenge successfully stopped a “flawed” process which would have blocked public funding for the treatment but now Sam and his family, backed by Leeds North West MP Greg Mulholland, are taking their case to Downing Street.
Yesterday Sam was joined by fellow pupils at The Whartons Primary School in Otley to design posters and banners and write letters to Prime Minister David Cameron supporting his case.
Early this week, Mr Mulholland led a Westminster Hall debate on drugs for ultra-rare diseases which highlighted uncertainties facing families due to NHS delays in the process.