HI A.,
I too have gone through the same scenario. The difference is that I've been on both sides of the story. My ex remarried a woman with two young boys... we have two girls. THey were 4 and 7 at the time. ALSO, I am a stepmother of two children. So I can see both sides of the issue.
I have to agree with the ladies that said you need to be making him go now. He is too young to make those decisions. If he is not expressing that he is being hurt physically or otherwise ,,and just doesn't want to go because he is mad or jealous. He has to learn to work out things that life hands you. We all want to protect our children from life,, but we can't. We have to teach them to cope.
You said you feel okay about this lady, so I think you should make it a point to discuss this with your son in depth. Make sure he knows that he will not lose his dad and that everything is going to be fine. Sometimes we have to just talk them through the problem.
It didn't sound like the dad was neglecting him in anyway, so Im not going to bad mouth him or be quick to make it all his dad's fault. So my guess is it's mostly the jealousy and the change.
Kids are tougher than we give them credit for. Just remind him he is loved by both and that everything will be okay.