Looks like you have a feeling that your boyfriend is playing a dual game. Talk to him than having benefit of doubts in mind. But your discussion to be up to the point and assertive in nature and NOT aggressive.
There are possibilities that he could have broken relationship with that girl and lied to you to win your love and confidence. If he is not in touch with that girl, you choose whether to let go of that thoughts and continue your relationship OR to decide otherwise . Itnisnnot worth living with undue doubts will mess your happiness.
Next Steps
If you have decided to accept him as your life partner, both of you can consider going for a pre-marital counselling.