child's mother is mia she has custody

Indiana allows parents to use either parent's address for school.  You may be able to register him for school.  After you have exhausted all means to contact mother and request that she consent to the registration,  I would contact the school and try to register him.  If the school refuses to register the child, you will need to file an Emergency Petition to Modify Custody and request an expedited hearing.  You have a significant problem if you are unable to determine mother's address as it will be difficult to obtain service.  The court is going to be reluctant to do anything if there is no service.  I would urge you to obtain legal representation.