Thanks for writing to us.
It seems that your son has suffered a soft tissue injury
of the chest wall. The soft lump is likely to be a hematoma
He needs a chest belt for proper immobilisation. Mild pain killers and hot fomentation in the area will be of help. You can consult an orthopedician to rule out ant fractured ribs in case the pain is persistent.
I hope this information has been both informative and helpful for you.
Dr. Rakhi Tayal