He was mentioned again in "Pipe Dreams", when Rita mentions his toothbrushes as a bathroom warming gift.
He was also mentioned in "Sister Act", when Rita took "Lana" (who was actually Lola impersonating her twin) to her dentist's appointment. After she and her husband learned that both twins had problems that should have been fixed during their appointments, Lynn Sr. decided to call Dr. Feinstein to tell him that he had missed some serious problems.
Dr. Feinstein is an elderly man, with very little hair and a white mustache. He wears a light blue dentist gown over a white shirt. Inside of the gown's pocket, he has two pens, one is green, and the other is red.
It is implied in "Dance, Dance Resolution" that Dr. Feinstein is of Jewish heritage, as the DJ in the aforementioned episode said that he was attending a Bar Mitzvah at Dr. Feinstein's.
A Bar Mitzvah is a ceremony held by Jewish people when a person's son has reached the age of 13, and are ready to observe religious precepts, and are eligible to take part in public worship.
In Rita's Listen Out Loud podcast, it is revealed that Dr. Feinstein attends a dentist convention called Cavity Con.