The bedrock of any relationship is trust, and this has been fractured by your partner's behaviour.
You have decided to give your relationship another go, and an essential part of that, is that you have to trust him.
If as he says, he is not interested, then nothing will happen, no matter how alluring his ex tries to make herself appear.
What you have to ask yourself is - do you really trust him, or are you going to worry every time he is out of sight?
If you cannot climb over the feeling that he will betray you again, then there is no future for your relationship.
You need time to heal, and get past this time in your life, and he needs to be very very supportive and understanding. If you do not feel these ingredients are in place - and they have to be to make a difficult time even possible - then you have to re-evaluate your chances of moving forward as a couple, or whether or not the damage is to deep to ever be repaired.