"Functional Neurological Problems" mean he recognizes your symptoms as neurological and that they are interfering with your daily functioning...pretty much what you you told him went you went to see him. Make another appointment and refuse to leave his office until he/she tells you what he/she believes might be the cause and what other tests might be done to determine the cause. Get/keep copies of ALL records from the hospital/doctor including doctors and nurses reports and notes and lab notes. If you just can't get these questions answered, go to another neurologist. Take your records. Don't assume the new neurologist will get everything (even if they use a common database of information). Sometimes even when the information is there at their disposal you need to bring it to their attention.
If you are ambitious, read the records. Look up what you don't understand. This is not to diagnose yourself but in order to be able to ask intelligent questions about your reports. If your doc is intimidated by that, get someone else. In my opinion, it's the dumb ones that are intimidated by intelligent questions. There are good docs out there that are not put off by your tactful, intelligent questions.
Neurological difficulties can be seen as frightening, even when there is no need to be. Have a good support system. Someone you can freely talk to about your fears and is encouraging to you...not just someone that cries with you.
I was diagnosed with cerebellar and cerebrum atrophy and still don't know the cause. (That doesn't mean of course that is your problem). I've seen three neurologists and I haven't given up. This newest one is great and feels sure he can find out the cause. Don't give up and don't let them give up. Be a squeaky wheel. As difficult as it is at this time in your life you have to be your own advocate. You have more vested in your body than anyone else. Hang in there.