I would tell your boyfriend what you are going through. He is going to find out sooner or later, when he does find out you want to be the one to tell him, and to tell him when you can have a calm conversation, not when you see your ex parked in front of your house and your boyfriend needs an explanation of why you are freaking out. Most men's initial reaction will be from the primitive part of their brain that is usually hardwired in, and that is to protect you. That isn't such a bad reaction, you should be prepared to be calm him down, you don't want him to over react and go get in a fight with your ex.

And if your boyfriend isn't supportive and protective, that's good information to know before you get too serious with him. Keeping secrets in a relationship is always a bad idea, and bringing things out into the light can often make dark scary issues seem less scary.