hello & welcome
acc. to symptoms, your son is suffering from some periapical infection.
Mode of treatment is the same as prescribed by your dentist
but as he is very young so it will be very difficult to do any of these.
secondly even roots are not completed at this age so even RCT is tough.
usually for very young child conscious sedation is given along with local anaesthesia, so you don't worry for that.
thanks & take care