I wish people would read the post properly! The gf is 14, it says so in the title. The OPs DS is also 14.
He could not be accused of statutory rape, nor will he be prosecuted.
They're experiencing all the normal feelings that two teenagers, raging with hormones have when they fancy someone.
I wouldn't tell him you've read his messages. The only time I'd do this is if I found something really worrying, like drugs, for example. Two 14 year olds, in a relationship, talking about having sex aren't doing anything wrong and shouldn't be made to feel that they are. If he knows you've read his messages, it'll completely break the trust he has in you.
However, I do agree that 14 is too young. I think all you can do is continue to have open conversations with him and try and encourage him to make the right decision.
I wouldn't tell her Mum you've read the messages, but maybe have a chat and say you've noticed the relationship seems to be getting more intense and you don't want them to do anything they could regret. Hopefully she's already clocked this and will be having similar conversations with her daughter.
At the end of the day, you can't stop them, but you can help them make the right decisions and stay safe.