When I think of prim and proper (for his mother), but still cute and fun (for you and your bf) I think of Audrey Hepburn's casual beach attire. Capri pants (or shorts, but considering that you're visiting his mom...) in neutral colors with striped shirts in bright colors and ballerina flats/sandals. I also think of knee-length or an inch higher sundresses in pretty patterns (floral!) and hats and sunglasses. Also, ditch the flip-flops and go for the low wedges or flat sandals. I agree, J. Crew and Lilly Pultizer are your best bets. Also, try to wear classic and basic things like simple solid-colored dresses (in white for day and black for night) but accesorize them with fun accesories (belts, shoes, headbands, hats, etc in fun colors). Also, a cardigan will help!
Always go for a classic style and never show too much cleavage or leg. Remember, the trick is to look classy but fun! Good luck!