I would not tell him until the time is almost up, then he will not have time to worry or be sad while she is still here.
Just tell him why she's leaving when the time is up; children get used to it quicker than we think when maids are leaving.
Our maid has been with us for 4 years and my 4 year old adores her. She will not be renewing with us as she wants to go into retail/something different. This won't be happening until Feb but I want to get my DS used to idea before it happens, ideas on what to say & when to approach it would be welcome before we decide how to go about it & what's best for him. TIA