The most common sites of pain are the fourth, fifth, and sixth ribs. This pain increases as you move your trunk or take deep breaths and, therefore, may cause breathing problems. Conversely, it decreases as your movement stops or with quiet breathing.
M and S have had pain in both places (on the side of their ribs and also at their breastbones) and so has my husband (those his was brief and very long ago). It's really very painful and miserable. Hang in there!
Currently have costochondritis and have many times before. It is quite scary the pain really mimics a heart attack. I find a heating pad helpful but otherwise it's getting an EKG for me to be sure and then waiting for it to heal.
My son has dealt with this for years. He doesn't have an arthritic diagnosis, but as you know we suspect that he does have some sort of arthritis. When his hurts, he feels that he has a difficult time taking a full breath. Does your ds feel that way too?