My son 3 had strep throat in november took amoxilin for 10 days now 2 months later is taking same med for scarlet fever. Could there be resistance?

Reinfected. You may have him enrolled in a day care and as such he probably became reinfected. Amoxil (amoxicillin) is still the drug of choice for strep infections or scarlet fever.
New infection. Not usually. I notice you child i three years old. It is quite possible he had another exposure to strep, possibly in a day care center.
Strep Throat. He may be harboring infections deep in his tonsils and if not resolved with antibiotics he would need his tonsils removed. Make sure he is on an antibiotic that covers anaerobic infections (augmentin, cleocin).