Contacting Potential Advisor After Being Waitlisted for PhD Admissions
You could but it's likely too late. The order of waitlist rolldowns is usually determined at the admissions meeting, sent to the graduate school, and is now beyond the professor's (and occasionally the department's) control. 

Departments and universities do vary in practice. There's nothing to be lost in emailing the professor but don't assume a response. No news would be bad news.