It's a very interesting condition. It's for life...once you have it, you have it. It typically goes into inactive cycles, with occasionally reactivation periods after medicating. Some people treated with Ganciclovir can develop a Ganciclovir Resistant form of CMV. That's when the Foscarnet therapy comes into play. Unfortunately, with the immunosupressed, CMV can have harsher effects.
The CMV can cause gastritis / Colitis symptoms. If this is his case, CMV of the gastrointestinal tract is typically short lived due to the constant sloughing off of the cells by gastro mucus.
With the immuno-suppressed, the data is harder to read since they have varying degrees of "compromise". Most did well though on Ganciclovir therapy, and went into remission.
Here's a good website that isn't too "technical" if you wish to read. http://www.healthline.com/adamcontent/cmv-immunocompromised-host
Pretty much, they'll need to boost your Dad's immune system back up while on the therapy to help the drug, and his own body, fight it back off. They will probably be cautious to put him back on an immuno-suppressor for a while.
(((Red))) Keep the faith. Knowledge is power. Trust that he is in the best care, keep his spirits up, and he will recover quickly.