Brain tumour is unlikely but a neurological cause can be there and a squint work-up is required by an ophthalmologist or an orthoptist to confirm that. Your son is more likely to have accomodative esotropia or acquired esotropia which are not present since birth.
Children who have been farsighted and have not had glasses, or children who were responsive to glasses but later developed an additional eye-crossing, are the most commonly affected with acquired esotropia. Children with acquired eye-crossing require prompt evaluation and treatment to correct the deviation and to restore binocular vision.
In accomodative esotropia, young children can often overcome farsightedness by focusing their eyes to adjust to the condition, but the effort required for this focusing causes the eyes to cross. Eyeglasses can reduce the focusing effort and sometimes straighten the eyes. You need to see a squint surgeon to confirm the diagnosis. Thanks.
Please, click the ACCEPT BUTTON so that I can get my dues. BONUS and a POSITIVE FEEDBACK will be appreciated.