Yes, this really could be a red flag!
: That must be like having someone tear your heart out! It feels awful to hold up this "I know what's good for you, dear" front while inside you're scared that you're sending your own child into a situation that merits his reaction.
I don't think any of us, including you, can make a decent judgment about what's happening at his dad's house or why your ds is doing this. It could be that he's just little, right? But you're his mom, and it's your job to make sure your kid is safe, and that what's going on is appropriate to his development. So whatever it is that's causing this, you have the right and responsibility to find out.
I'm just saying all that because it seems like you might be hesitating to make any moves to disrupt the visits. But this situation is definitely unusual. Most kids love their daddy, no matter what. A lot of kids that age do favor one parent over the other for a while, but that's when both parents are available. And this is escalating. So refusing to let him go over again by himself until this is cleared up would be very reasonable to me.
If it were me, I would call an agency that deals with social services. Is there a mental health center? I would start by calling the food stamp office to find a place that offers councelling to families as a social service. If there isn't anything in your area, I would call cps. Not to send the dogs on them, but to have your concerns voiced within the "system". That gives you support if you need to go to court, and it means that there will be some faster action if your son is actually in danger.
let us know what happens!