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.