I think the first thing that may help you is to understand your son's behavior isn't rational, and can't be interpreted that way. OCD, while in part behavioral, also involves brain chemistry that makes it difficult to make the behavior changes. Because of the medication issue, it's not easy to correct that for your son. (and even without the epilepsy I've know parents just as frustrated with their OCD young adult children who are still living at home)
You really need someone who knows all the cutting edge treatments - first, the medications that might not interfere with the epilepsy, and someone who can do more than just basic CBT, although that's part of it. It very well may be that your son ends up with a defined disability, but that could be a good thing in some ways as he may get medical insurance, and some basic living support. But he also could continue to work as much as he's able. But please try to understand this isn't your fault, and that you can't cure it, as much as you understandably want to.
Please take care of yourself. It does sound like your son has a number of skills and that he may need to his hit bottom before he can make decisions for himself. It's actually tough for all of us to accept our bodies aren't working the way they're supposed to. As someone who's lived with a seizure disorder for over 40 years, I know the "oddness" (can't think of a better word) of knowing that your body periodically does things all by itself that all you can do it stand by and watch. My grand mal seizures have been under control for decades, but I do get minor jerking periodically, especially when I don't sleep well (which happened when I wasn't sleeping because of pain). Then I tried some pain meds that nearly caused another seizure, so had to stop them. OCD is kind of like that with the mind, I think (although I don't have firsthand experience with OCD). All of your sons urges aren't under his rational control, and he probably is confused about that himself.
Is there a major teaching hospital in your city that specializes in complicated cases? Because that's where you might find the right specialists in the same group. Epileptologists, neuropsychiatrists & psychologists, etc. and have your son get a thorough eval. so he could know his options. Then if he chooses not to take advantage of them, you need to let him experience the consequences. That's where you'll need support with your anxiety.
I hope this helps some.
Please remember to find me over at the chronic pain forum if needed!