I would say that the obvious first answer to your question is BECAUSE IT IS A DRUG! Perhaps they did not wish to subject their son to the side effects of Gleevac or any other cancer drug. Gleevac side effects include:
Fluid retention
Swelling in tissues
Fluid in the lungs
Nausea, vomiting, diarrhea
Muscle cramps
Bleeding
Changes in the blood counts
Decreased liver function
(http://www.swedish.org/13499.cfm)
They may also have known that the Gleevac success rates for their son's particular kind of cancer may have been much, much less than 99% - and, while Gleevac may have had 90 plus percent "positive response" rates in some trials, nowhere does it say that Gleevac has an actual cure rate of 99%.
In fact, the Journal of the National Cancer Institute reported this year that:
"Although complete hematologic remission rates occur in more than 95% of patients, only a minority entera molecular remission in which BCR–ABL is undetectableby polymerase chain reaction. In acute phase, characterizedby an increase of immature blast cells in the peripheral blood,only about 70% of patients have a clinical response, and mostof them relapse within a few months."
Nowhere in the story can I find what kind of cancer or leukemia the boy had. CML or AML? Or another of the 12 major forms of Leukemia? The high results reported for Gleevac apply to early stage CML and may not apply to other forms of leukemia. Skeptics have countered that "the simple genetics of CML created an ideal situation for this kind of therapy--one mutation, one mutant protein, one drug target--while most cancers are more complex medleys of mutations"
Also I could not find what stage his cancer was in - crucial information in most instances. According to the FDA:
Gleevec has been show to produce responses in 31% of patients in blast phase and 93% of patients in the chronic phase who are not responding to interferon. Gleevec specifically targets the mutation that is characteristic of CML leukemic cells, and although remissions are very likely, the long-term durable remission rate and cure rate is uncertain.
(http://www.swedish.org/13499.cfm)
In the final analysis, all cancer drugs are poisons. Taking them is making a very shaky bet that somehow you will poison the cancer more than you poison your own body and that you will not later die as a result of the damage you have caused by ingesting something nature never intended to be placed in your body.