Hi there and thanks so much for posting this question, please feel free to post in the Forum Neurology or similar. From your description anything can be, normally behavior is associated to an efficient communication between left and right brain hemisphere, that is why people suffering bipolar disorder they refer to abnormal behavior and problems of left and right brain communicating effectively although is not the only reason.
However may be stress and anxiety as well that sometimes pushes us to perform unwanted actions, some of us can control some of them can't and there are sometimes that there is no clear diagnose.
My recommendation will be to get him do Meditation and Physical exercises every day one in the morning and once at night and make sure he does them, I have plenty of exercises in the internet just google my name and ask me any questions, we are trying to see if regular exercise will improve behavior I know it does but it takes time to see the first results so I am wondering if that is a possibility. Don't worry about what you mentioned that one of the brain hemispheres is underdeveloped it is clear that may be the cause but thanks to the neuroplasticity of the brain there is so much you and he can do to increase that plasticity and better communication between left and right brain hemisphere.
I hope this helps!