I'm not speaking of a CAT scan... The test I am speaking of is a nuclear medicine test where red cells are 'tagged' with a radioactive element and your son would be put in a detector-scanner.... but not a CT scanner, and if red cells are leaking somewhere in the GI tract the detector will give the doctors an idea of where the bleeding is. HERE IS A GOOD REFERENCE FROM THE MAYO CLINIC
A splenic problem is possible but unlikely based on what you have told me. Anemias caused by splenic dysfunction will usually produce abnormal red cells on the basic blood count... not just a decrease in the cells.... but abnormally shaped cells. Here is a link to learn about the tagged red cell scan.
You are wise to wonder about an iron absorption problem. I think that this is less likely than slow and undetectable blood loss in the GI tract. It is usually possible to tell on the simple blood count if the anemia is due to iron deficiency or blood loss versus the malabsorption anemia called pernicious anemia.
Something uncommon that could cause this kind of picture would be a MECKEL'S DIVERTICULUM
The good news in all this is that we know your son's anemia is responsive to iron supplements. The other good news is that he is going to see a hematologist, and the hematologist is going to figure this out.
Sometimes the end of the diagnostic road in anemia involves a bone marrow biopsy, but I don't think this will be needed in your son's case.
Be sure to have a list of questions to ask... Here would be my top questions.
1. Do we know if this is a blood loss anemia, or a malabsorption anemia?
2. What is the best way, diagnostically, to get to the answer?
And once you get the diagnosis, the question is, obviously, is if the condition is curable or merely treatable on the long term.
Please see the Mayo clinic link above on anemia... it is well written and easy to understand.
And as to a problem of isolated iron absorption, no, unless his diet is simply lacking in iron then this is not the answer.
Also please let me know if you have any further questions. I am going to bed now but can answer in the morning.
If this has been a good answer for you, positive feedback is appreciated.