– Jason Leider can no longer hold a pencil. The 9-year-old is down to his last 20 words, and the only needs he can express are “hot dog” for hungry and “towel” for cold.

“If he could just go back to where he was,” said his mother, Deena Leider, remembering a younger Jason who knew numbers, shapes and colors, who could jump and run and play baseball.

Jason has Hunter syndrome, a rare, incurable genetic disease that in its severe form — which Jason has — causes progressive brain damage in patients until they descend into a vegetative state and die, usually in their teen years. A little more than a year ago, Jason’s IQ was in the mid-50s. He could scrawl a “J” to sign his name, eat on his own and tell his dad he was thirsty. His IQ has dropped 10 points and he has lost motor and mental skills.

Recently, Jason and his family were at the University of North Carolina at Chapel Hill to receive a drug that his parents hope will not only stop the disease’s deadly advance, but prolong his life and even recover some of what he’s lost.

His dad, Jeff Leider, said he’s never known the real Jason because “he’s been masked by this disease.” With the treatment, he said, “I’m finally gonna meet my son.”

Jason’s doctor, Dr. Joseph Muenzer, who is leading the clinical trial of the drug, idursulfase-IT, said it “appears to be beneficial in helping stabilize the disease,” offering hope that victims of the illness could live longer, though how long isn’t clear. The drug, which replaces missing enzymes in Hunter patients, is still being tested and hasn’t received government approval.

“The key is prevention,” Muenzer said. “Once you lose brain cells, you’re never going to get them back.”

Jason flew to North Carolina last July and barely qualified for the trial with an IQ score of 56. To his parents’ devastation, he was drawn into the control group, which is composed of patients who aren’t given the drug so that their outcomes can be measured against those who are. Patients in the control group were promised the drug after a year. Jason’s year is up.

In November, Jason’s brother, Justin, 6, a first-grader whose Hunter syndrome is less advanced than Jason’s, tested to enter the trial, scoring just below the upper limit of 85 on the IQ test to qualify. Justin began getting monthly doses of idursulfase-IT in November.

Jason and Justin will receive injections of the drug until the clinical trial is over, around January 2017, Muenzer said. If the results show “significant difference” between the treated patients and the untreated patients, the drug could get federal approval and enter the market within months, he said.

In the nine months he’s received the drug, Justin has shown no signs of deterioration, his parents said. “He’s in there somewhere,” Deena Leider said. “He’s just completely clouded by the disease.”