Welcome @jsmith1006, I know it is extremely difficult to watch your child in pain and not knowing how to help them. It sounds like his pediatrician thought it might be related to growing pains and referred him to a pediatric rheumatologist. Then your son's pain went away and came back after 5 months. I'm hoping members with similar experience will be able to respond with what they found out. I did find some information on growing pains but not sure how helpful it is.

"Why is my 4 year old in pain at night?
Growing pains often strike in late afternoon or early evening before bed but pain can sometimes wake a sleeping child. Most kids are pain-free and active as usual during the day. Growing pains may last for months or years, but they don't happen every day. There might be days, weeks, or months between pain episodes."
-- Growing Pains (for Parents) | Nemours KidsHealth: https://kidshealth.org/en/parents/growing-pains.html

Mayo Clinic has a symptom checker that might be worth trying - Joint pain or muscle pain in children: https://www.mayoclinic.org/symptom-checker/joint-pain-or-muscle-pain-in-children-child/related-factors/itt-20009075. Mayo Clinic also has a Children's Center if that is an option - https://www.mayoclinic.org/departments-centers/childrens-center.

Have you contacted your son's pediatric rheumatologist to see if they might have some suggestions for a follow up appointment?