There are a number of reasons that your son may be experiencing this weight loss and the bump, which may or may not be related. This amount of weight loss, while dramatic, may still be due to a simple viral infection. I would agree with you that this should be evaluated rather than just waiting to see what happens.
The bump may be from some trauma as you initially suspected. Often, the initial bang isn't even noticed until the bruise/bump shows up later. It may also be a lymph node or a small fat cell tumor (non-cancer) called a lipoma.
Having your son seen by his doctor is a good idea to be sure everything is still OK.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.