Is there any legitimacy to this report of planet nine being discovered by an amateur?
This has not been confirmed so whether it is legit is unknown at this time.  A recent report (https://arxiv.org/abs/1706.05348) calls into question the original analysis that leads to the initial report of planet nine. The report that fueled initial interest is given below.

(EVIDENCE FOR A DISTANT GIANT PLANET IN THE SOLAR SYSTEM
Konstantin Batygin and Michael E. Brown
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; kbatygin@gps.caltech.edu
Received 2015 November 13; accepted 2016 January 10; published 2016 January 20)