I don't know how to deal with a toxic ex-husband?

We have an autistic son. And it requires me to attend IEP's with him. My son lives with him because the school district in my town was not helping him at all.  My son is much more happier in his new school.  And we travel back and for to see him.  When he is with me my ex comes to visit him and when he is with my ex, I go there to visit him.  I don't know what to do.

I divorced 6 years ago. My marriage was not a good one. My family is in another country and my family was him and my son. I observed that when he was droping my son in my house, because he lives very far, he was staying the night and leaving in the morning. I did not like it until I told him that he needed to leave. He did. Until the last two times that my son has requested me to let him stay. The second time he did stay, he started kissing me and hugging me, he had tried before but 

 I always pushed him and aggressively have told him to leave me alone. This last time, he told me that he thought we have a chance, and he had change. I told him that it was weird, then I agree to kiss him, and we did kiss. Then the next day I called him and I wanted to discuss what happened. He told me that he doesnot know. That he does not want to get married again, that he knows we are not a good match. That we can date but not kissing and nothing intimate. That he was lying, and he just do

he just does not know what is going on with him. I don't know what is going on with this man. Why does he need to lie, why is so hard for him just to be a friend? I have to have a relationship with him because of my son, but it is very hard to me because I want us to be friends. What can I do?

I have to deal with this toxic guy. That I do not like, but he is an excellent dad. And our son loves him.