Thanks for your query.
The painless lump could indeed be a collection of blood (due to rupture of muscle vessels); or, rupture of few fibers of the neck muscle, due to muscle sprain
If the lump is due to a collection of blood within the muscle, it will resolve on its own within 3-4 weeks' time.
Now that your son is all right, except for the lump, there IS NOTHING SERIOUS to worry about.
Consult your doctor and apprise him of my opinion. I am certain that he agree with me, order required tests, and- based on the reports- will prescribe suitable treatment as advised.
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Fond regards and have a nice day
Dr. Rakesh Karanwal