Being a Assistant professor of Zoology requires the eligibility of NET qualification along with a PhD (mandatory for Universities) or MSc (some colleges do recruit). Now for MSc, there are not much need of Physics as elgibility criteria for Zoology. So your brother, isn't expected to face any problem laterwards in his career.

For more information regarding Zoology consider the link below:

https://www.careers360.com/courses/zoology-course

I hope this helps. Wish you a great day!