It is difficult to say for sure because there are many factors that will affect the final outcome. The crime of impersonating a Peace Officer is a class D felony in Kentucky. A peace officer is defined as any member of a law enforcement agency, so probation would be included in that.
Class D felonies are punishable by one to five years in prison however this charge is frequently negotiated down to something else like disturbing the peace. He is likely going to do a little bit of time in county jail due to the fact that he was already on probation. If his record was totally clean I could see community service or a work program. Good luck.