“He’ll have lunch”, simply means that he will eat a meal, but saying “He’ll have lunch” we are not entirely sure that it will happen. Future Simple is used here like a prediction about what may happen in the future. Once the future is fully imagined, it will be used in the future.

As b is the author mentions a particular hour: 12:45, we use Future continual to emphasize the activity ( at this time he will be eating his lunch, he will be during it) in

the sentence “cdb3iu” below.