Hi Dip,
Illness often does cause a drop in hemoglobin level, so this may explain the drop. Hb levels do vary a bit and are affected by the season and also by hydration. A well hydrated person might actually show a lower Hb even though there is no problem, so small changes should be expected from test to test. A physical observation of the child has to be included in any analysis of how well the drug is working. If growth is normal, activity level and appetite are good, these factors have to be taken into account as positives when hydroxyurea is involved.
If possible, L-carntine and magnesium should be taken with hydroxyurea to help maximize its affect.
It can't be predicted if he will need a transfusion in the future, but as long as the Hb doesn't drop below 7-8, depending on the observed health of the child, transfusions should not be needed.