Hi July,
40 mg seems like it could be a bit low for a burst for a 16 y/o. When I did my first steroid burst, it was 50 mg for five days, and I weighed 99 lbs. I’ve also done steroid tapers, and those usually started at 60 mg. Every one is different, though.
That first time I did a five-day burst, I saw improvement on the third day. I’ve had other flares when it took a few days, and still others when I improved within a day on steroids. There have also been a few when the steroids didn’t help at all, but that was when I had infections that I didn’t know about.
If there’s an active infection, it’s entirely possible that the steroids will either not work, or they’ll make him worse. Have you looked into Lyme disease through Igenex testing yet? It’s so important to treat Lyme if it’s there—otherwise, a lot of the PANS treatments won’t have their full effect. Does he still have his tonsils/adenoids? Infections can hide there, too, and antibiotics won’t reach them.