Final answer:
John's surgeon should avoid using the skeletal muscle relaxant Succinylcholine.
Step-by-step explanation:
Based on John's medical history of having a family history of malignant hyperthermia and a genetic disorder causing low atypical pseudocholinesterase activity, his surgeon should avoid using the skeletal muscle relaxant Succinylcholine. Succinylcholine is contraindicated in patients with these conditions because it can trigger malignant hyperthermia and cause severe adverse reactions due to the low pseudocholinesterase activity.