Zoladex will eventually fail to keep the cancer in check, but it's not due to losing control of testosterone - normally Zoladex will keep the testosterone very low anyway. What happens is that cancer continually mutates, and eventually by chance it generates a variant which doesn't need testosterone to multiply, so the growth of these new cancer cells is not kept in check by the absence of testosterone. These cells are referred to as "castrate resistant", meaning they will grow even if the patient is castrated (the old treatment), or still on Zoladex (more commonly used today).
The oncologist will look at things like the staging and the length of time it takes the PSA to double to decide how quickly the cancer will spread and impact Quality of Life. It might well be that this is going to take sufficiently long that even unchecked, your father is likely to die from something else first, in which case there's no point adding PCa treatments from which your father will gain no benefit. On the other hand, it might be that adding another drug is necessary to slow the PCa down to the point where it's unlikely to be the cause of death.