I can't offer parent-of-teen advice, but, I can offer more-recently-teen-then-the-rest-of-you advice, in regards to things teenagers say, and how my parents (ineffectively) dealt with me (ignorance. As a lesbian, they just hoped ignoring it would make it go away, and hey, there aren't gonna be any babies).
The fact that he's talking to you is the major thing. Like, that trust is great so you do have to give a little back.
Do the whole safe sex and appropriate relationship practices talk, highlighting that you don't think he should be engaging in any behaviour of that sort, but that if he does you want him to be safe, responsible. That goes beyond condoms, but y'know, how to treat a girl and stuff.
In all liklihood, they'll be an 'item' and go on 'dates' to the cinema and hold hand and kiss, but him having said the 'keep it mates' thing? It's probably not gonna go much further.
I know being 14/15, me and my friends had 'partners' all the time, but nothing ever really went on. It's just how you title mutual likings at that age, and it doesn't hurt your reputation.
Conversely.... they might want to have sex. Outline safety, mild disapproval, and not in your house. If they're gonna do it they're gonna do it, and there's nothing you can do to stop it. Keeping communication is key, and as you would with a girl-child, make sure he knows he can talk to you, that he can say no, that if anything makes him uncomfortable he can stop it.
In regards to modelling.... Eh. It depends on how serious a model she is, and the demand she faces. He'll deal, with the aid of the almighty Skype and Facebook.
From your point of view, its probably ideal if they'll be apart more- nothing physical can go off, and it will either strengthen the relationship into something substantial, or they'll realise they're just good friends.
And, be nice to and about her. Invite her for dinner, try to not embarrass both of them, even affectionately. Set parameters like if they're in his room the door has to be open, or they have to stay downstairs or whatever.
I know this isn't exactly what you were looking for, but, I know what definitely wasn't a good approach, and I know what would have been nice to have recieved from my parents in regards to my relationships when a teenager. And, I'm pretty familiar with how teenagers think and what tends to go off.
Teenagers are pretty resiliant. You have trust and communication, and that's the best thing for influencing and protecting any part of your small ones life.