Aching Legs Hodgkins Lymphoma I Have A 3 Year Old Son Who Keeps Complaining Of Aching Legs?

I have a 3 year old son who keeps complaining of aching legs? - aching legs hodgkins lymphoma

I have complained to 3 years old son, the pain in the legs, in particular a knee. Especially after walking a lot and sometimes it seems that the weakness in the legs. Any idea what this could mean