Heaven knows what I'm doing up this late scanning the epilepsy board, but here I am. It doesn't seem like it's just the epilepsy that's challenging your son, but you seem to know that. I don't know if your son is working with a psychiatrist and a therapist, but that might help to evaluate and treat other issues - such as possible OCD. He's been functioning ok it seems for a number of years, but something is changing for him. And it does seem like he will need to get connected with some type of facility and/or group living situation that helps him manage his affairs sometime in the future.
One thing that might help you is a different therapist. Someone you could talk to who would help you not just with your son, but to make plans for your future that would ease your anxiety. Perhaps some of this could be done in a will, and that would ease your worries of the future.
It's late and I hope I've not said anything that's upsetting, but maybe something I've said might help.