"Mom, I don't want to go see my dad"
Hearing that just breaks my heart I am so sorry you have to hear that. I know you want your son to feel positive about going to dad's and you want him not to see that it bothers you that he doesn't want to go but it may behoove you to at least ask your son why he feels that way...
I am now trying to get around the whole joint signature for my daughter to see a therapist. Certainly, if your ex can take your son to an accupuncturist you can take him to a therapist. It might help him to be able to talk to someone impartial so he can deal with the feelings he is having as a result of going between two different environments.
You just never know what may be going on over there and he may not be old enough to put it into words for you yet. I have not yet heard back from the therapist I called on Friday (she checks messages Mon-Th but when I do I will let you know what she says.
Hang in there mama and don't let his dad interfere with your child having adequate and consistent childcare! That's definitely crossing the line.