I would imagine your son had a low renin reading, which indicates he doesn’t need quite so much fludrocortisone.  Fludrocortisone contains a certain amount of glucocorticoid (hydrocortisone) and so by lowering this, they are in effect lowering his hydrocortisone dose a little bit too.  I expect they will then check his bloods to see what changes lowering the fludrocortisone has done and will then lower the hydrocortisone too, if results show it is still too high.  Changing both medications at the same time is not a good idea as it is then difficult to see the effect each individual change has made.  Hope this makes sense!