Tibia Pain, Pronated Foot, Pain Not Frequent. Reason?
Thank you for posting your query.
The area you have described is where the Patellar tendon is located. May be your nephew is right because mal-aligned biomechanics of the knee have similar symptoms. But whether you pronate your foot excessively, knowingly or in subconsciously, needs clinical evaluation.
The other reason could be Epiphysitis of the insertion of patellar tendon, or early arthritic changes, inflammation of the anterior fat pad in the knee besides a variety of other conditions.
In my opinion, it looks like a harmless inflammation around your knee that will subside with rest, hot fomentation, local ointment and support with a crepe bandage. Avoid medication as far as possible. Use proper footware and avoid climbing stairs for a few days. If this does not subside have yourself clinically evaluated by your orthopaedician.
Hope this helps you understand the situation. If you need to undertand anything else, I'm available for follow up.
Good luck.