Hello and hope you are doing well.
The lump behind the ear could be due to mastoiditis, which is infection of the bone. This can happen with middle ear infections. It could also be lymph node enlargement. Lymph are protective mechanisms of the body and they 'trap' the infecting source causing enlargement of lymph nodes. The infection can be from the draining skin or the organs. Treatment is usually with antibiotics and anti-inflammatory medications. So, consult your primary care physician for proper clinical assessment.
Hope this helped and do keep us posted.
Well, this could be a number of things. His Pediatrician should be notified.