it is a personal or private type of question and when you ask something like that you are crashing into a woman's personal space and her personal life where you don't belong.
you can equate it with going up and putting your hands on strangers. i'm sure he wouldn't like it if people he didn't know walked up to him and began touching him. and just as he can't do that to others, questions like "are you pregnant" has the same type of effect.
bottom line: it's none of his business if some woman is pregnant. maybe explain there are certain types of things he doesn't have to know about other people--things that are none of his business. and a woman being pregnant is one of them. and, very importantly, you told him not to do it.
also--he's 7 years old. he's not an idiot. i'd angrily explain to him a bit more and then tell him to knock it the hell off or suffer the consequences. sometimes kids think it's funny to push our buttons--you've told him not to do it and he keeps doing it. maybe he's doing it on purpose and is amused because he keeps getting away with it--you can't stop him. maybe if you raise your voice and look mad he'll take you more seriously.
i work with a lot of kids and have had the annoying experience of 6 and 7 year olds tapping my breasts to get my attention or just because they felt like doing it. i've also experienced younger kids spanking or hitting me on my butt because they think it's cute or clever. needless to say i address the situation very seriously and explain that it is not appropriate for them to be touching grown-ups like that. and if they repeat their behavior i tell them i will be having a talk with their parents about it. very often it appears the kid knew exactly what he/she was doing, understood they shouldn't be doing it, but decided to do it anyway because they felt they could get away with it.
next time he pulls it maybe he could lose a privilege of your choosing (video game, afternoon with friend, tv show, lunch at mcdonalds, cool thing he wants for school) and when he protests just remind him of his rude behavior and inability to mind you.
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