your husband is having low iron saturation in blood, which indicates 3 things
1. his dietary iron intake is not adequate which is most common but unlikely for your husband
2. he is having defective iron absorption from his stomach.
3. he is having some systemic disorder which is impairing his iron rserve.
to rule out these you have to wait atleas for 6 weeks on iron rich diet so as to improve iron SAT values in the blood. so wait till then if he is not having any other significant difficulties other than constipation
which is likely to develop with iron rich diets(no need to panic).
after that do the following tests to confirm the diagnosis... serum FE , Iron SAT, blood Routine, peripheral smear, USG abdomen, serum creatinine
note: if possible kindly post the entire blood profile of your husband i will go thru it and get back to you