How much time must my child spend on his phone?

See below. He does not have to spend any time on his phone. You may want to set reasonable limits, such as no more than 30 to 60 minutes a day.
Depends on age. Under 1 yr of age it's best for a child not to be on the cell phone. Over a year, if for using apps to communicate like "FaceTime" that is less problematic. Certainly there are some educational programs that can be accessed for kids. In general no more than 1-2 hours per day (even for teens) is advised. For very young kids, try to limit to 20 min/session + take breaks as eyes are still developing.