Cindy,
His pain management is a little unorthodox. There are 2 types of pain management: "Short term" and "long term" and the opioids are distinctly different for either indication.

"Long term" opioids are prescribed when an individual is antipated to be in pain some a relatively long period of time - a month or longer anyway. These will take the form of a time release tablet or patch and will provide continuous pain relief to manage the pain threshold to a tolerable level. Typical meds would be Oxycontin or Fentanyl (AKA Duragesic) Sometimes they even prescribe Methadone.

"Short term" opioids are prescribed for pain lasting less than a month or for "breakthrough pain", when the pain threshold exceeds that managed by the "long term" opioids. Typical "short term" opioids are Morphine Sulphate, Codeine, Percoden (AKA Oxycodone)and it's weaker brother Vicodin (AKA Hydrocodone).

It is quite normal to have both types of opioids available at the same time.

Orally taken, Morphine is the fastest acting, reaching full effect in 30 minutes. The other "short term" meds take about an hour for full therapeutic levels to be attained.

Post op patients will often be given a Demerol or Morphine drip by IV which they can self administer with a pushbutton (dose rate and intervals regulated by the equipment to prevent OD), but here again this is a 1-3 day short term application and IV reaches the full therapeutic dose instantly.

Here is a link to the NCCN guidelines for cancer pain management
http://www.nccn.org/professionals/physician_gls/PDF/pain.pdf

He is very vulnerable to infection while his immune system is compromised. You were wise getting him to the hospital immediately. The rule is any fever exceeding 100.5 degrees requires an immediate ER visit. His temperature should be taken several times a day. A log should also be kept so any trends can be identified.