I'm using Minoxidil but doctor said when I stop using it, my beard will fall out. Is this true?

The simple answer is yes but it’s really not so simple. If you are between 18-22 there is some likelihood that minoxidl will accelerate at least some of the pathways that lead to fuller facial growth you are destined to have by age 30 (when facial hair is complete). Some hairs will be minoxidil dependent but not all. Most young men don’t return exactly to thr same pre-use levels but yes there is some loss.

Any positive effects you get from using minoxidil you will lose if you stop.  You won't be worse off than you would have been, it will just be like you'd never been on it in the first place. 

If you are using it on your beard hair and had regrowth from it, then yes stopping the Minoxidil will cause those hairs that grew from it to shed.  Good luck!!