I understand how hard it can be to get a new pdoc in some areas, so I guess you just have to deal with this one. I would tell him I am confused and I would like to see some documentation concerning dossing. Remember, the dose is agreed to by you and the pdoc, not forced upon you.
My pdoc writes with a very heavy hand and I have disputes over doses all the time with her. On lamictal she has me at 400, but has mentioned I could go to 500. When I told her I thought 400 was the limit she said she knows of cases that are at 500, even 600. Remember as I said she is very heavy on doses, but even she never said that she has heard of 800.
I wanted to be sure what I was advising you before I did, so I looked at
Lamictal Dosage - Drugs.com it is a very informative piece and says NOTHING about a 800mg dose. It looked to me like an optimum dose is between 200 and 400, depending what other drugs you take.
Since you cant get a new doc, and you have to work with this one, do some research you can quote and then be firm, he can't force you to take that large a dose.
I really hope you get things worked out on this, nothing worse then a pdoc pushing drugs on you that you don't want or need.