My 14-year-old son avoids socialising with groups his own age

I would like guidance on my 14-year-old son’s massive social anxiety. He is at the point where he won’t engage with anyone in his peer group or with siblings outside the front door.
When his 15-year-old brother’s friends drop into the house, he will remove himself from the livingroom so he does not have to talk to them. Despite this, he is full of chat within the family circle at home, so that is not a problem. He is also happy to work part-time in a job that does not demand interaction.
However, when it comes to being social with groups his own age, it is an absolute no from him. He can throw a wobbly when we push him. We are worried about him missing out on making friendships with kids his own age.
Now that he is back to school, I am not sure what to do.
Many young teenagers find it hard to fit in with friendship groups of peers their own age. There are lots of different reasons for this. Young teenagers who are discovering their own identities can be very judgemental of each other. In peer groups there can be narrow social rules about what is acceptable and navigating group banter and group jokes can be stressful for many teenagers who are struggling to fit in. This is especially the case for teenagers who might be neurodivergent and who communicate differently or who have different interests.
Group bullying peaks in the early teenage years and this often leads to some teenagers feeling excluded or isolated. Further in the school environment, teenagers have less choice about the friendships group they can belong to, which is different from when they are in college, when they more easily choose groups they more easily connect with.
It might help to reduce the pressure on your son to socialise in peer groups. It is okay and understandable that he might not want to be social with his brother’s friends when they visit. Even if they are only a year older, this would be intimidating for most teens. Also, his brother and he are probably different in how they socialise and this is okay. Also, it is your son’s choice at the moment that he does not want to meet peer groups outside home and that he might prefer being at home instead. Celebrate what is already working for your son. It is great that he is “full of chat” at home with his family and also that he is holding down a part-time job at 14 (even a non-social one).
Rather than trying to change things, take time to understand what is going on for your son. Do you have a sense of why he avoids peer groups? Or do you know what makes him anxious or what he specifically finds difficult? Or do you know what he loves to do and when he feels the most happy? Often, it can be difficult for young teenagers to talk directly about what is going for them, so often you have to be a bit of a detective to discern this. They might talk more openly at certain times, such as when you are doing something alone together or over a favourite meal or late at night if he is in a good mood then.
Try to identify these good communication times so you can make sure to be available to listen.
Be careful about imposing your own goals on what you think he should be doing socially and instead discover what he wants. Listen carefully to see if there is something he would like to change or something he would like support with. For example, he might be open to talking through his friendship dilemmas and you can be there to listen and support him. Or you might be able to help him practically. For example, rather than meeting in larger groups, you can facilitate him meeting peers and friends one to one by taking him to good places to meet or giving space for him to invite them home.
Setting small goals and going at his pace is likely to be more successful.
Informal and unstructured social situations can be the most anxiety-provoking for many teens. As a result, many find organised and well supervised activities where there are clear rules a much easier place to make friends and to learn to socialise with groups. There are many different types of activities that could work for your son, such as team sports, scouts or drama or more niche activities such as debating, quizzes, computer/gaming club or choir.
The key is to help your son find activities that he enjoys and is good at, when he is likely to feel much more comfortable and to meet children like him. This will make socialising much easier for him.