I’m sure this is extremely difficult for the whole family. I doubt it is inherent anxiety, since the behavior only occurs when you are present. But there may be something about your approach or presentation that is making him anxious. Children, especially sensitive children, pick up our feelings regardless of how well we think we’re managing.
Unfortunately, this type of situation can take on a life of its own. The more you try to manage it but without success, the more anxious you are liable to get. The more anxious you are (even if you do your best not to show it), the more a sensitive child will pick that up and become anxious as well.
I’m very happy to hear that your marriage is solid. When parents are mutually supportive, it helps both maintain consistency, which is what your little guy needs the most. Unfortunately, I can’t offer suggestions about what you could change without knowing you better.
For that reason, I suggest you consult with a family therapist — not because I think there is a deeper problem but because a family therapist could observe how you and your spouse interact with your child and could then make concrete suggestions about what to do differently.
The best book I’ve found for parent education is a very old one. It’s called Children: The Challenge by Rudolf Dreikurs. Dr. Dreikurs offers a simple way to understand children’s misbehavior as well as concrete suggestions about how to deal with it. He also provides lots of helpful examples.
Please don’t feel bad that you haven’t found a way to solve this on your own. Managing two children is different than managing one. It’s not at all unusual for couples who did fine with one child to be challenged by two. There are now four people in your family who have differing needs and sensitivities. I hope you will consider getting some advice by someone who can take everyone’s personalities and needs into account.
I wish you well.