My Son, Age 44 Is Having Severe Joint Pain Mainly

My son, age 44 is having severe joint pain mainly in his legs and feet, ankle area. He has not been able to walk for the last 2 weeks. He has been to the emergency room twice and they have given him some pain medication. All his blood work for Lymes Disease have come back negative. Don't know what to do next. His PCP isn't doing anything.