Hi
Thanks for choosing HCM for your query.
As per the features described in your case I am of the opinion that your son is having a
popliteal cyst popularly known as Baker's cyst.
It is caused by some underlying disorder of the knee such as a
meniscus injury resulting in increase in
synovial fluid (natural lubricating fluid) in the knee joint.
It may also be due to a torn
cartilage meniscus in the knee joint due to traumatic injury.
Treatment:
It is usually self limiting one.
Avoid any activities which aggravate or make the condition worse, so rest is important.
He can wear a compressive wrap (crepe bandage) around knee support to help reduce the swelling.
If swelling does not clear up on its own then you are advised to visit an orthopedic doctor for proper assessment.
Other causes of the above mentioned problem can be foreign bodies or sometime an abnormal dilation of the blood vessel (
aneurysm) that has to be ruled out.
Hope your doubts are being clarified.
Wishing your son a smooth and quick recovery.