It is very important that you read the leaflet included with the prescription. Watch for signs of overdose or side effects. Call the pharmacy and explain what happened. If they are closed then call the pediatrician's office - they almost all have an after hours answering service for situations like this. The doctor or nurse should be able to tell you what to watch for. There's also poison control at 800-222-1222.

This is from DailyMed's listing on Bromfed DM (same ingredients as what your son was prescribed along with Dextromethorphan):

"Central nervous system effects from overdosage of brompheniramine may vary from depression to stimulation, especially in children. Anticholinergic effects may be noted. Toxic doses of pseudoephedrine may result in CNS stimulation, tachycardia, hypertension, and cardiac arrhythmias; signs of CNS depression may occasionally be seen."

Hoping that your son is okay!