A Baker's cyst is a pocket of gelatinous fluid that forms a lump behind the knee. It is also called a popliteal cyst
Such cysts are commonly seen in children and are different than in adults.
They are usually asymptomatic, and are not related to serious joint abnormality.. Spontaneous resolution usually occurs, although the process can take several years.
The diagnosis may be confirmed by ultrasonography
(to differentiate a solid mass from a cystic lesion) or aspiration.
In most cases, these cysts should be left alone, as they often resolve spontaneously.
Surgical removal of a popliteal cyst is indicated only when symptoms are severe and limiting and have not resolved after several months.
Getting an MRI
done will confirm the diagnosis & will also rule out any serious pathology that might be presenting as baker's cyst.
Hope this information is helpful .
Wishing a speedy confirmation of diagnosis for your grandson.