Depends on what he's doing to try and hookup and if he'll get spiteful if you reject Him. Is he the type that will spread lies and tell your partner that you were sleeping with him just to cause doubt and problems with your partner? So ,if he is ,then you should tell your partner because if he finds out that there was any type of communication and you hid it from him,that could cause him doubt and he might start to believe you ex's lies. You know the kind of person your ex is. So only you can decide what the risks for telling or not telling your partner. But don't give him even the smallest reason to doubt you if your ex plays dirty. Good Luck
You are required to determine if you are okay with it or not. Inform your ex-husband on your progress and ask him politely to back off. If he gets too disturbing, you can inform your current partner to stop him.
Do what you believe would be best for you, if the situation persist therefore intimidating your daily life or affecting your current relationship it is advised you take action. Letting him know you have now moved on past is past all is well and you do not need him in your life, when you have now found a significant other. Good luck