"He" or "it" are both possible, it depends of a context. If it's "it" there should be used some other verb like "start"... For example: "I'll let you know when it (the film, the lesson...) starts."
The meaning is "when he leaves/it starts then I'll let you know". It's not like "I'll let you know when he/it is supposed to leave/start."
I think you can accept my first suggestion and the other one (with "it" you can put in remarks field.
Hope I helped and didn't make it complicated.
Please ask me if there's anything you don't understand.