Is it okay to mention the potential advisor's name in my personal statement? I have already established contact with my potential advisor. Would it be appropriate to mention his name in my personal statement? Would doing so be looked upon negatively by the admissions committee?
It's perfectly acceptable to mention a potential supervisor's name, especially if they already know who you are. Indeed, many places that I applied to specifically requested that I include the name of a potential supervisor, either in the personal statement or elsewhere on the application.

In doing so, the admissions committee can see whether you've done your homework on the research that goes on at their institution (for example, it's no good writing down the name of someone who works on observing supernovae to then say that you're interested in theoretical research on modified gravity models).