Alternating hot & cold is a totally legitimate and effective modality when applied intelligently & consistently.
The real advantage of alternating hot & cold is that it creates a sort of pumping action, allowing nutrients into the damaged tissues at an enhanced rate/volume & damage debris out; this helps get around swelling, splinting & other adaptive mechanisms. This therapy was a major factor in my getting the (*ahem*) upper hand on arthritis.
Rather than write an entire brief on the benefits & use, I'll just ask you about the nature of your dad's problem: where in the back? What kind of pain (aching, throbbing, piercing, crushing, etc)? Who recommended hot-&-cold? How long has he been suffering? How did he injure himself? What else has been tried (& hasn't worked)?
Meaningful suggestions must wait for more detailed data.