How much could I talk with him?

It'd be nice to know why you chose your guy over the friend you had fall for you, but my answer basically remains the same.

The first thing you need to do is basically establish some ground rules and borders that your friend needs to understand. You can do this all very gently, you don't have to jump on the poor guy and beat him into the dirt with it. *But*.

He does need to understand that you're not available at the moment and you're with your current boyfriend, and be able to respect that boundary. He also needs to understand that him continuing to pursue you and flirt with you has the potential to cause problematic drama with your current boyfriend. If he really does care about you he should be able to respect these boundaries pretty easily and understand why they have to exist.

There's nothing wrong with keeping a healthy amount of comunication going back and forth between you and your friend as long as things don't get out of hand and the needed boundaries are respected. If he's unable to accept the idea that he has to respect that you're with someone else right now, you unfortunately may have to tell the guy that you just can't continue speaking with him or seeing him anymore because its putting to much strain on you, your relationship, and your friendship with this guy.

Hopefully this helps.