Welcome to NT....

You are free to Google B12 anytime, and find reputable links to satisfy your doctor. I have some on this thread in fact.

You can follow your doctor's advice, and see if you get worse, or no better, and then decide 1) find another doctor, or 2) take control of your body yourself. When it comes to B12, this supplement is very safe to deal with yourself. This is not true for everything in life however.

We would like to think in a perfect world, our doctors know everything that will help us and make us better. But the truth of the matter is that they do not. In a decision like this, you need to consider if doctor's advice is better than your body's needs.

Do this-- make your doctor prove to YOU that a serum level of B12 at 2000 is harmful. I'd like to see what he/she comes up with!

In fact lab ranges are arbitrary. They are taken from volunteers who appear healthy, and averaged. They are not based on people who are supplementing anything. Extremely high B12 in a person who IS NOT supplementing, may point to some rare conditions. That is all they mean.
And furthermore, what is very lame about the US, is that the LOWER end of the range is out of date, compared to other countries. Normal is considered 150-200 here and in Japan anything below 500 is abnormal.