My son is 6 years old and was bring up in the environment similar to yours, of course, he also got all the symptoms that your sons have.
The reason was not known until he entered Y1 in an international school in 2008 and he was recommended to attend an Occupational Assessement done by an Occupational Therapist. The report shown that due to my son have had the problem of "Sensory Integeration" and thus, he was so sensitive to the environment including the stranger and adult. After several months' SI training, his situation is much improved but still have rooms to improve especially he would take a longer time to build up the trust and feeling of secure with the adult including the class teacher.