If you express your concerns to the photographer she/he has tricks up their sleeve regarding posing and lighting.
I would avoid turtlenecks, do a v-neck and simple, classic necklace to emphasize your bust. No patterns, solid colors. I'd avoid a babydoll tee like the plague, not only is it trendy (date your portraits quickly) they are unflattering. If you do go with it, remember not to squish your arms against your body - it makes them appear twice as large.
Maybe a solid sweater of a nice skimming, flowing fabric like cashmere? I don't like synthetic materials in photographs - too shiny. Try the sweater idea to match textures with him, it will pull your look together as couple better. Look for a drapey or v-neck neckline. Perhaps 3/4 sleeves. I think greys go well with maroon and look nice on everyone, but that's just me (a portrait photographer
ETA: This sweater from Victoria's Secret sort of match my description. Not exciting, classic style...
I like how they flow, rather than cling. The second one is great for emphasizing your bust and minimizing your middle.
It also depends on the nature of the portrait. Is it formal or casual? The best bet is to dress along the lines of your BF or make him dress along the lines of you - not matching outfits (oh, no
) but matching taste and style.