Thank you. I have some ideas that might help your son.
When it comes to talking with other people, many people experience fears. One of the most significant is not knowing what to say. This also creates the fear that others will feel you are uninterested or even socially awkward. The best way to approach this is to have in mind at least three topics that fit any social situation, such as current events, social events involving the group and something about the group you are in such as something someone is talking about that you can comment on. Practice talking about these topics with "safe" people like family or a close friend.
Another fear is that people will notice how nervous you are. To deal with this fear, it helps to confront it by asking yourself "so what?". What can happen if they notice? Everyone has been afraid at one time or another so realizing that can help you feel less nervous. Changing your thinking about it helps.
Take along a comforting object, one that gives you tactile sensation, like a smooth stone. It can help to play with this object as you talk to not only distract you from feeling nervous, but also to calm you.
When in doubt, bring up something about the other person. A simple compliment can break the ice and make the other person perceive you as approachable and kind. People love to talk about themselves so whenever you feel stuck, ask a question about that person. Learn their interests and ask them about it when you see them, like "how was the game?"
Social anxiety often develops from the belief that you are not as good as other people. Working on increasing your self esteem can help a lot. There are numerous websites and books on it. But the main idea is to start believing you are just as worthy as others. You can begin to work on that right now by using positive statements to change your thinking. It sounds simple and it does take some time, but it works. Leaving a note to yourself on your mirror or carry it with you can help you remember. Something simple like "I am worthy and loved" can make a big difference in your self esteem.