Paul Rutherford, grandfather to Warren Todd, a 14-year-old boy with a rare genetic disorder, knows that David Cameron must have an inkling of what his family goes through every day.

Warren is one of only 100 known cases of Potocki-Shaffer syndrome in the world. The condition affects every single part of his life.

Like David and Samantha Cameron’s son, Ivan , who died at the age of six in 2009, he needs round-the-clock care. Ivan lived with cerebral palsy and a rare and severe form of epilepsy known as Ohtahara syndrome.

Warren also has epilepsy, added to severe learning disabilities and skeletal problems.

His grandparents Paul and Sue can only imagine the pain the Camerons went through when they lost their “­beautiful boy”.

That’s why they make their offer ­carefully and without malice.

“We want to ask David Cameron to come to our home and visit our family,” Paul, 56…