Thanks for the query to H.C.M. Forum.
In my opinion your son has received shot known as benzathine penicillin 12 lac or 24 lac unit intramuscular injection
He is right the progress of this disease will halt at once and according to his treating doctor repeat of injection is necessary after 15 days or 30 days for some time.
No side effect of this injection except pain.
If he got another shot than benzathine penicillin I don't know about that.
Good luck. Dr. HET