I started out with double vision. My MG blood work was Negative. I went to a neuro-opto and he dx'd Mg and put me on a large dose of prednisone and it cleared my double vision in a few months. My blood work came out positive 6 months later. My MG has progressed to generalized. Mestinon never helped my vision problems.
I suggest he see a neuro-opt.
Mike