Do you know how he and his father are getting along and what activities they have together? what happens when he's with his father. Does he stay over at his Dad's place. If so that is a big change for a little guy. I would try making the visits to not be overnight and see if that makes a difference.
Have you talked with the father about what is happening and if so does he have any ideas. Will he describe his time with his son for you? If not perhaps he'd talk with someone else.
Have you asked your son why he's feeling this way? What does he say? Does he talk about the time he spends with his father?
I agree that you should get him into play therapy. Often kids are able to express their feelings thru play and a therapist can interpret the play and recommend ways to manage the behavior. You definitely need to do this now before a negative pattern gets set at school.
Another option is to ask the school district to evaluate him for a learning disorder. He could be having difficulties with sensory issues which is causing him to misbehave. Federal law requires school districts to provide such evaluations and treatment for free if treatment is seen as necessary to have success in school.
Some of this need to control is normal at this age. He's becoming aware that his life is changing and in some ways he has more control which makes him more aware of how little control he has. He's testing boundaries to find out where he can have control. And having his father come back into his life, while good in the long run, does add stress to his life.
It is also not uncommon for kids this age to have nightmares. Our dreams are a way for us to work out feelings and things happening in our lives. If you don't hold and comfort him when he wakes up I suggest that you do that. He needs reassurance that you're with him and will always be there to be sure he's safe.
I would call the therapist back if you don't hear from him in the next couple of days. I would also be sure to give your son lots of love and approval. Praise him for everything that he does right even if you have to reach somewhat. Tell him he's your good boy and give him hugs just because it feels good to do so.
And find a way to relax. This is a stage in your lives and will be over with some professional help.