4 yo son recently diagnosis with strep (+ rapid) fever and sore throat are getting better after 24 hrs of abx. no cold symptoms but now has dry cough; related?

Maybe, maybe not.: Typically strep throat alone rarely produces cough--there is almost always something else involved when a cough joins an infected throat such an a sinus infection or 2nd illness (not previously appreciated until the strep begins to improve) such as mycoplasma. The antibiotic for strep will not treat mycoplasma infection; therefore, the best advice is to call your md noting this change in symptoms.