Plz help Low haemoglobin count after age of 55?

My father has a very low HB count as low as 8 and 8.5..in his peak years he used to have it as 14-16. He has been donating blood till he was afte he was 40.but notmuch after that..and not at all in the last 7-8 years..Please suggest tests to detect teh reason for low HB..we have tried iron capsules 1 daily but dsnt help much.what all tests we should go ahead for .Plz help