M.G.
I COMPLETELY disagree with the idea that the more you tell him not to go out, the more he will (I've seen what those kids do). He is THIRTEEN years old, and YOU are the parent. It is absurd to use that mindset for that age. If you do that, then you are saying that he is the one in charge. You and your husband need to first get together and find some middle ground because he'll play you two against one another if he knows you disagree. This is the age group I teach, so after being around them every day for years, I can say that I hear things and see things, and they just want some ground rules. No, they may not like them, but this is the time they test your boundaries to see what they can or can't get away with. If you let him tell you what he's doing about dating right now, then oh my goodness what will he do when he's 16?
Having said that, you also have to let him feel like he is the one making the decisions. For example: you could give him the choice of continuing to use his phone (which probably has all the cool features) as long as he's not contacting those girls or receiving calls/messages from the girls OR you can get him one of those other phones where you program who can and can't get in touch with him. But it's his choice, and you could give him maybe a week to decide which he would prefer. The thing is to make it not sound too authoritative because this is the age that they really want some independence as well as to know that you trust them. He is no longer a child, but he's not grown up yet. He needs something to show that he is his own person. Just let him know that it is really cool that so many girls find him so interesting, but he needs to understand that you are only concerned about where it is going. Maybe if he wants to have a few of his age friends (boys and girls) come over and hang out, they could do so in your view so that he knows you're still all for him having friends and fun. There are just limits to what he can do with his friends.
Kids are doing things so much younger now that I would have never dreamed of and it's scary, so this is the time to set your limits, let him know what they are and stick to them but not in an "in your face" manner if that makes sense. I hope this helps, and I commend you for standing firm on what you've already told him. way to go!