It depends on your current relationship and your relationship with your ex. Depends If you and your current boyfriend are in a serious relationship, depends on if you know he talks to girls and his exes in a friendly matter. Just think about it and if you think it's a bad idea. If its just friendly conversations he probably would understand.
It depends on the understanding between you and your boyfriend. Have both of you vowed to not speak to ex boy-girl friends? If not, then why would you tell him? If he somehow found out and asked you, tell the truth. If you have a commitment though, think twice, and perhaps think about why you want to talk with your ex.