Gymnasium Admission

In general, the parents decide which type of secondary school a kid enrolls at. The school only gives a recommendation based on the kid's skills - for foreign kids there may be an assessment test (ask at the school authority of your town).
But if you enroll him in a Gymnasium (the highest kind of secondary school, leading to univerity admission) without the necessary preconditions, he might fail there and be downgraded to Realschule or Hauptschule (the lower kinds of secondary school) after two frustrating years of repeating failed classes.
Therefore most teachers would recommend starting at Realschule and, if he is good there, upgrading to Gymnasium (which is always possible once the preconditions are met).
B1 German is not enough to sufficiently understand lessons, so you may explore the option of enrolling him in a preparatory class for foreign students at first. These are offered at some schools (again, the school authority can tell you which ones) and, although it means he will graduate a year later, greatly reduce the chance of failure and demotivation.