When I was 11, I was 'dating' 14 year olds, however, that's not to say I think it was a good idea. I was lucky that they were people from my school, and not random creeps (not saying that your ds is a creep! But just saying it was likely the only reason I was okay!) this was only around 5 years ago, so not something that could have changed with times. Personally, I think it depends on the situation, when I was 13 I dated a 15 year old who turned out to be very narcissistic, and abusive (sexually and mentally), but then I have also dated a 14 year old having just turned 13, and it was okay. It really depends on how far they're planning to go with each other, as most of the 'relationships' I'm talking about here were holding hands, pecks on the lips when our friends weren't there to tease us, overall very innocent, whereas when I dated the 15 year old it was sexual, proper making out etc, and for a very long time too (well, for teenagers! It lasted about a year and a half). I'm sure you know your son, and his maturity level, if he's likely to be doing anything like that then I'd step in and end it, if he's more 'young' for his age (not that that's a bad thing!) and is the type to blush after a peck on the cheek, then I would say it's okay, and just monitor it. I'm only a bit older than your son, and while I wouldn't want to date a 13 year old for example, I am an 'old' person for my age, and sadly much more mature than most, which I wish I wasn't.
However it's not just down to your son, he could be the loveliest boy but if she's a manipulative girl then stuff could not work out anyway, so I'd want to know more about her and perhaps meet her, but this could happen with a girl his age too, but I'd be more worried because of the age difference.
All the best 