Obviously he is the only one who knows where his heart lies and if he is with his ex. You do have to consider that if you are having this dilemma on a regular basis then this is always going to be a stressor in your life if you are with him. You have to consider whether that is something you want to tolerate over time. The fact that you wonder if he is with his ex proves that this is always going to be a stressor. For that is change he would need to develop strong boundaries with his ex and with your relationship. He can be a father without being a partner. You would need to make it clear that this is necessary and why.. If he can't do that then he can't be a good partner.
He may not have done anything intentionally but instead does not have firm boundaries with this ex and that is leading to these difficulties. He may be letting the ex run his life by holding the children over his head and that causes him to bend every time she calls. That would need to change.
As far as contacting you it seems that you both have to define what you want from each other. This off and on again relationship is only leading to heartache. If he is done he needs to tell you he is done. You can't change him until you decide where this is going. It is rude and insensitive to not return your calls so you need to decide if this is another behavior you wish to tolerate. Maybe he isn't' ready to commit to you because he has no boundaries with his ex.
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He will have to learn. The timing of this probably was just wrong and he hasn't managed that. I would have been wary too. If he is insecure then he probably did find it difficult to let go. Just keep in mind you would have to deal with these insecurities over time. There would have been issues.
Be back tomorrow