Question: I am mentoring a teacher, who has massive discipline problems. A lot
orginates in his being utterly borring. So it's a personality issue. What
can I do?

Answer: Wow! what a problem.
Does he think that he is boring?
What kinds of activities would he suggest that he would think that his students would enjoy?
Is he able to recognize good activities that would get a fair amount of students' interest?
You might look at some ideas on motivation and see if his instruction provides sufficient areas of interest for the various variables that would get students motivated. However, probably the first important one would be the tasks/ activities that students select or are given to do.
I don't know how helpful that is, but it might give you some place to start. If you could provide some more information, I might be able to give additional suggestions.
Or maybe, as the tone of your email suggests, if it is a hopeless endeavor, you could drop hints of other job opportunities or leave the want ads laying around. :-)