I've suffered from social anxiety since I was very young and have never sought help for it until now (22 years old) .my advice is to get your son help now!!! through your GP, because I really wish my parents had done the same for me and I've only ended up resenting them for not helping me sooner.
Social anxiety is an inherited mental health issue which affects physical, emotional and mental aspects of a person. As you are aware, if your son is suffering from anxiety, your son will not outgrow it nor will it go away. What to do - first you need to see your family doctor or pediatrician - if he/she is not experienced in anxiety issues, then request a referral to a medical person who will be able to help you as a child neurologist, child psychiatrist, child psychologist, etc. Treatment will consist of intervention, therapy and possibly medication. For such a young child, I expect intervention may be all that is required. Since each child is different, you will need a professional to guide you down this path. By the way, the earlier the diagnosis; the better the prognosis (and three years of age is not too young to treat this issue). In addition, we have found from our support group (I belong to a support group for teachers and parents of children suffering from anxiety) that the younger the child; the shorter the treatment.
You might wish to google terms as "childhood anxiety" or "social anxiety in children" or similar phrases to see what information you can find on the internet. Social anxiety can be debilitating to the child and yet, it can be highly treatable. In addition, you will find a wealth of information on this topic in bookstores or in your local library. And yes, anxiety does interfere with learning - we found from our group that this "difficulty in learning" became particularly evident when the child began Grade 3 (I suppose before Grade 3 much of the information the child required could have been mastered in the comfort of home). Some of our anxiety-prone children who have not learned how to manage their fears often "shut down" in class and are unable to learn very little in school. Please contact your family doctor as soon as possible and if you feel that I might be able to help in any way, please feel free to contact me. I wish you the best ...