Your son has a habit which is involuntary! We do have some habit breaking appliances , but at 2.7yrs , he is too young . It could be a temporary phase and just wait for it to pass. Many kids develop different kinds of habit which dissappear with age.You can get a check up done at your nearest centre to have a detailed look if you want.
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As a mother of two sons, my advice is just wait for sometime, this shall pass.