My boys are a similar age and whilst I would imagine that this new relationship for him is nothing as serious as requiring a talk, we have never really hidden the facts of life. We have always had kids books about sex as well as books about how our body works etc available in the bookcase for them together with books about dinosaurs and anything else they might be interested in, we've answered questions as they came up as well.
I maybe wouldn't go with a full blown talk, but maybe try to suss out how much he is aware off when you are chatting to him in general about his new girlfriend.
I don't know if it just coincidence or not, but my two don't really seem particularly interested in the opposite sex yet. They both had girls they liked when they were 5 or 6, but seem to be more interested in lego, fighting and star wars at the moment!