Irobot ..." Sunny... Different From The Rest..."
Hi there and welcome to the forum...
Think most of it has already been said by now but first impressions last, and be well dressed ( not a short dress ) as I expect his mother is the one who will have a unspoken private view of you at the time of first meeting you like...ohh she`s a nice girl or not, for some reason but that`s mum`s for you...
Be polite and honest and a small gift of flowers for his mother and a bottle of his fathers favourite tipple (drink ) maybe, and your in for life...
Your other half should of sorted his parents out way before this as you said you have already met his father who probably has said how nice and wonderful you are but this meeting does sound a bit Too formal... as they already know who you are and why you are there....The classic meet my Girl Friend / Fiancée is a UK statement of I am Serious with this woman my intentions are..... to his parents.
I think my approach would be to my parents would be along the lines of ...hi mom / dad here`s ?????? then make a pot of tea and have a casual chat..... Phil...