I'm with Alibaba - I don't think there is much need to worry. When I was pregnant with DD I was told a maximum of 6 cups, but they seem to have reduced that recently.
If you are going to be addicted to something then it might as well be something that isn't actually bad for you. If he starts drinking 15 units of alcohol a day, that is time to worry.
I must admit I don't understand the point of giving up caffiene, unless it disagrees with you and you have bad dreams or something.
I probably have about 4, on average.