“What is the best medicine for a child's sinus infection?”

My response is to ask you why you think your son has a sinus infection, and how is he? Children have different sinus development than adults, and a bacterial sinus infection is notoriously over diagnosed. If he has symptoms like fever, colored nasal drainage, facial pressure, congestion lasting greater than 6 days without improvement, then maybe it's reasonable to diagnose him with a bacterial sinus infection. But any improvement in these symptoms within 6 days is considered viral and should be treated with supportive care only.
Nasal saline, short acting decongestion spray for 3 days, hydration, even a steam bath can be helpful. Nasal steroid spray regularly for 2 weeks is helpful.