Hi thanks for your question.
It looks like that your child has developed a heamatoma(collection of blood under skin and muscle) following trauma.Since his ultrsound is normal there is nothing to worry,The injury
might be superfascial one.Haematoma
has given him swelling.I would suggest you to consult a nearby physician who may prescribe him pain killer.He may also need antibiotic.If you have no access to a physician you can give him pain killer available over the counter like paracetamol
.As his appointment with urologist
is still 4 days to go and he is in pain and agony.
Hope this answers your question.