"I Don't Like Them" & reverting back

I just got licensed last week and I am waiting on my first placement so I do not claim to be an expert. However, my son only sees his father sporadically and has been in therapy because he struggles with anxiety and with dealing with being abandoned by his father. I will see huge improvements and then set backs after visits. I imagine this is similar to foster kids having visits with birth parents. The more loving an understanding I am despite his behavior the sooner he gets back to his normal self. part of it is being so young not even understanding their own feelings much less knowing how to express them properly, but also it is a test. For my son it is something like this though he can't put it in these words, "Dad left, if I am bad will mom leave or is she gonna prove she loves me no matter what." I have no foster experience but thought this might help a little.