Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. It seems while your grandson was given an allergy spot his muscle/tendon got hurt/injured. The muscles responsible for the flexion of the forearm are brachiradialis, brachialis, Bicep brachii. This flexor contracture and may cause pain later. Visit an ER & get him examined. He needs medications like tramadol or some muscle relaxant. But first the degree of hurt should be assessed. Hope I have answered your query. Let me know if I can assist you further.