Others have more expertise than myself about this. My guess is that it is safe to take this for now, but that is only a guess. Later, he can re-evaluate. Maybe it is fine for the long term.maybe he needs to add some things...(magnesium and B12/mentioned below)

Can your brother get his vitamin levels tested?
I think the B vitamins have helped me a little with my facial nerve pain. I take a B multi and extra B 12.
I also take a good overall Multivitamin. Another supplement I think might help is magnesium. I've heard it said that magnesium relaxes muscles and it might have some benefit for nerve pain. I've also heard that it is common for people to be low in magnesium intake. And b 12 often isn't absorbed well. These are two important supplements, possibly even more important when a person suffers from pain, yet oddly typically low in many adults.
There are many old posts about these supplements, especially magnesium. This product you've listed doesn't seem to have magnesium and I've heard it's always a good idea to take calcium with magnesium and again, magnesium is often low in adults and possibly very important for those in pain. It is either impossible or practically impossible to take too much B12, so no concerns there. What is in this supplement you've listed is very low. Methycobalamin seems to be the preferred form of B12. This is what I take.

So, my thought is perhaps to take this product for a few months AND add magnesium and B12. Perhaps during this time, your brother can find a doctor with expertise regarding supplementation and his personal needs. He can also use this time to evaluate and research. But, he needs to start somewhere.

Sorry to repeat myself, but I think eating well, lots of vegetables and avoiding sweets and junk food is important. This way, many nutrients can be attained naturally. And foods that potentially can inflame the already damaged nerve would be avoided. Believe me, this has been a very difficult journey for myself.

https://ods.od.nih.gov/factsheets/Ma...hProfessional/

http://neurotalk.psychcentral.com/sh...ght=Vitamin+12

http://neurotalk.psychcentral.com/thread85103.html