AKT-0028

A 5-year-old boy is brought by his mother into the out-of-hours (OOH) walk-in centre. She reports that he is thought to have an allergy to peanuts, and is waiting for an allergy clinic outpatient appointment. He has eaten a piece of birthday cake at a party around 30 minutes ago, and quickly developed facial flushing, with swelling of the lips and face. He has become wheezy and is now unable to talk in complete sentences.
You suspect anaphylaxis.
What is the most appropriate immediate management option?