Hi, Well having cyst in the psoas major muscle is actually too less to hear about. May be your son has over tightness of the psoas muscle. this can be helped by performing the stretching of the muscle and correcting the Imbalance with in the muscle groups. The exercise can be figured out from the internet as well. But as your physician asked you to meet s sports therapist , you can surely meet up as it will increase your confidence that nothing is major with my child. Hope I have answered your query. Let me know if I can assist you further. Regards, Jay Indravadan Patel, Physical Therapist or Physiotherapist