I still talk to my ex-boyfriend and my current boyfriend doesn't know.
That's a little risky. Why are you still talking to your ex boyfriend?
He used to be my best friend and we have a lot of great memories together.
It can be great to catch up with friends, but that becomes complicated when your friend is also your ex. You should be honest with your current boyfriend and let him know that you're still talking to your ex, otherwise he may get jealous if he finds out.
Why would he get jealous? My ex isn't flirting with me and I can choose to talk to him if I want to.
It doesn't matter if he won't get jealous or not, I'm just saying that it can make things muddied up. How would your boyfriend feel if he found you're lying, he might think you're getting feelings for your ex again.