It's normal for teens to mess up. In many different forms. Have you sat him down and told him everything you told us? He's 13, so he's old enough to understand all the worry and angst that is going through your head, and even empathize with it. Tell him this is your first time parenting a 13 year old and you're trying to figure it out too. Make sure he knows the logic behind your rules and the consequences for breaking them.
Also, since he's 13, his brain is starting to marinate in hormones, plus his higher function judgement and decision making skills aren't developed yet. All that will make it harder for him to say no to friends- especially the ones he has a crush on.
Biggest thing though- make sure he knows that you think this is a one- off for him. That you love him and think he's a great kid who just messed up. Come up with consequences and an action plan to fix things together. This shows him you view him with respect and as an adult-in-the-making. And adults fix their messes.