Jonathan Simms, 18, from Belfast, who was struck down with variant CJD a year ago, had an infusion of a drug previously tested only on animals injected directly into his brain.
The treatment at a British hospital, which cannot be named for legal reasons, came only after his family fought a battles in the High Courts in London and Belfast.
No cure has yet been found for the disease but Jonathan’s father Don said they cannot give up trying.
“It is our only hope of saving our son. The best we can hope for is that the disease will be slowed down or will clear from the brain completely.
“We just don’t know what will happen until it is tried. We are relieved we have now got this far but at the same time we are frightened because we are stepping into the unknown.
“My son is not being used as a guinea pig. Without this he has no chance of survival. This is an experimental treatment but it is not experiment for experiment’s sake.
“We have been looking after Jonathan around the clock at home and we have been watching him dying slowly before our eyes. We know that we are doing what Jonathan would have wanted us to do if he was able to communicate with us.”
Mr Simms, an electrician from the Highfield estate in West Belfast, found details of the experimental drug pentosan polysulphate on the internet.
He campaigned tirelessly to get the right to use the treatment on his son. There was never any time that we wanted to give up and now we can only wait and hope.”
If Jonathan survives the first infusions of the drug, the doses will be gradually increased. But there are fears that it may cause further damage to the brain and hasten his death.
The Simms family will have to foot the cost of the treatment which is likely to run into tens of thousands of pounds as well as heavy legal costs for their court battles.