Hi.. Can understand your concern.. As per your complain in case of throat pain the common cause is either a bacterial or a viral infection.. In case the strep test is negative then it is more possible to be a viral infection.. As amoxicillin is an antibiotic and is taken to treat bacterial infections and if your son is having a viral infection antibiotics will not help in relieving the symptoms as antibiotics doesn t not act against viruses... Although at times antibiotics are given to prevent additional infection to occur by bacteria. Amoxicillin gargles are not commonly prescribed but the doctor must have prescribed it so that it can act localized over the throat and does not go into the body as there is no bacterial infection found.. So antibiotic gargle will just help in keeping localized throat and mouth bacterial count low and prevent any infection.. I would suggest you that make your son do warm saline gargles as it can provide relief.. Throat lozenges taken every 4 hourly can be soothing to the throat.. In case of pain Ibuprofen can be taken as it will reduce pain and inflammation.. Avoid spicy and hard foods. Hot milk with a pinch of turmeric boiled in it can be taken bed time for three to five consecutive days can be helpful.. Hope your query is solved.. If you find the answer helpful please write a positive review and click on find this answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora.