Hello and welcome to HCM
Their is nothing to immediately worried about the lump. But you needs to be investigated it so that it does't harm in the future.
If the lump causes pain, increase in size with time or cause deformity
in leg or cause any problem in movements of leg than you have to get check it as soon as possible. As you have explained in your mail that your son has non of the above said symptoms so your can wait one week for your appointment.
The diagnostic test for the lump are FNAC test (needle aspiration test) or in some cases MRI
will help to get to a diagnosis. Consult your doctor and follow the advises give by him.