A 4-year-old child from Delhi presents with a 5-day history of sore throat and fever. Throat culture on Loeffler's serum medium shows grey-white colonies. Gram stain reveals Gram-positive bacilli arranged in a Chinese letter pattern. Elek test is positive. The child has already received 40,000 units of diphtheria antitoxin 24 hours ago and is on penicillin V. Today, the child develops acute onset of weakness in both legs with inability to walk, and mild ptosis. What is the most appropriate next step?