That is not normal in any way. Take him to your pediatrician, tell them what you suspect and the reasons why and the pediatrician will have to report it to CPS in your state as a required reporter. Be prepared to provide whatever contact information you have for "mom" to the pedi at the appointment and any law enforcement agencies that contact you. This gives you a little cover from any fall out within the family -- I'm assuming you may be concerned about that given that your first line mentions seeking advice from outside the family.
Did "mom" give you any formal paperwork establishing your custody while she's getting her life straight? At a minimum you should have something which authorizes you to seek medical care on his behalf and a copy of his health insurance card, immunization records, contact info for regular pedi... My gut says you don't want to take him to his pedi and its better to go to one you know & trust -- its possible he hasn't even been regularly to the pedi so "mom" could hide what was going on.
I'm so very sorry for him and your family. Please make an appointment immediately with pedi to investigate possible abuse, a child mental health professional to help your nephew, and a very good family lawyer to initiate emergency custody proceedings.