Neuropsychological Testing (96118)

Hello, I am doing medical billing for a clinical psychologist and he does testing quite frequently. I am used to billing auto for that testing and I always combine all the hours spent on the test and bill as so many uits; for example, if he spent 15 hours on the test, I bill it at 15 units once the testing is complete. My boss has billed it differently where if he spent 3 hours one day, he enters 3 separate charges of the 96118 on the same date at 1 unit per charge. I do not think he is doing it correctly and wanted to ask your opinion on what is the correct way to bill this.