I have a lot of sympathy. My DS2 has just turned 14 and has had a girlfriend for 9 months. Her family are very keen on encouraging the relationship, and invite him to family events etc.; I think they are too young and want to keep them as 'just friends'.
That said, I know I can't be an ostrich. :) I have had 'the talk', about sex and about love. I've talked to him about his feelings and bit my tongue listened rather than just lecturing him. That was actually reassuring, because it told me he isn't remotely interested in sex - yet - and that he knows about contraception and where to get condoms, if/when he does get interested. I told him I'd take him to the FPC whenever that is, and that I'll talk to him about anything, any time. I also made sure he knows I think that under-age sex isn't a good idea, and that it isn't generally even enjoyable because young people's bodies and minds aren't ready and they're often very anxious.
I've also set some ground rules - well one main one, which is that the door stays open if they're in his bedroom together.
I have made a point of talking to his GF's mum. I wanted to check that she was happy with this rule, and that she'd enforce it and supervise them generally at her house. I agree it sort of legitimises the situation and makes it 'serious', but I would far sooner do that than have him become a father at 14. And that is a real risk - you can't avoid it - a boy who isn't informed about sex, love and contraception is much more likely to get someone pregnant. And young people whose parents are avoiding the whole issue are more likely to find themselves in difficult situations they can't handle, without support.
I think you just have to be brave Aix, and talk to him. Good luck!