There are a few possible reasons for this. First, it may be that his anemia was from something else. This is probably less likely since iron deficiency is by far the most common cause of anemia and his iron levels are low.
It may also be that he has received enough iron by this point to make new red cells and recover from the anemia but not enough yet to restore his iron stores. In this case, he simply needs to remain on treatment dose iron for a bit longer.
The other possibility is that he is continuing to loose iron. The most common reason for this situation would be blood loss from the GI tract. There is a simple test to look for blood in the stool that may be a reasonable test to do.
Follow up with his doctor will help assess this further.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.