Breastfeeding is NATURAL and nothing to be ashamed about. I think that you should BF in front of your son so that he can see that this is how babies are meant to be nurished. It is important for him to learn that breasts are not soley a sexual object and what the true purpose of them is. Also, seeing breastfeeding as normal and just how mom cares for baby will increase the probability that he will encourage his own wife some day to breastfeed his children.
That said, he may be uncomfortable with it at first - simply due to the whole preteen emotions. But maybe talking to him first, showing him some discreet pics of nursing moms, and even a trip to a LLL meeting may help him (if that is okay with him of course). ANd if *he* chooses to leave the room when you nurse the new baby - that is okay too. But I would not force him to leave - that sends the message that breasts and breastfeeding are sexual and dirty. Which is so not the case!! Go slow, talk to him, and give it time - Hopefully he will eventually feel comfortable with it.
I have breastfeed in front of my husband, my father, my brother, my FIL, my BIL - becuase *I* don't have a problem with it and *I* don't view it as anything sexual, and honestly if they do they can leave the room. Not one has - although my FIL raised some interesting questions about size and color differences of nipples!

That said - I dont' strip down to BF DS - I am discreet, it is hard not to be - no cover up needed cuz nursing bras and shirts cover it all. The only time a nipple is ever really exposed is getting DS latched on or when he pulls off and wants to play around. While DS is latched on, you can't see anything - while NIP people have thought he was sleepign in my arms, but he was wide awake and nursing like a fiend!
