When you say your husband's cancer has come back I am assuming that he has had a PSA test and the reading is higher than 0.1 and therefore not classed as undetectable. I am no expert in all of this and only have my own experience to go by but as you are worried about what the oncologist might have to say I will try to give you some reassurance.
If the PSA was not undetectable following the op it means that there are still some cells left. They could be in the prostate bed or elsewhere and so the onco needs to explain this and what the options are for further treatment. They might suggest having more PSA tests to see if the number rises and how quickly as this can give a guide as to where the remaining cells are. They may suggest starting hormone therapy which can weaken or kill off cells wherever they might be or they might suggest radiotherapy if they feel that the cells are in the prostate bed as RT needs to be focused.
They might suggest HT and RT.
There are many combinations to consider but it is still early days yet and nothing to panic about.
I think the important thing is to get the onco to explain the situation clearly so that you feel comfortable that you know what is going on. Once you understand where you are I am certain that you will feel more comfortable with discussing the next steps.
All the best to you both.
Kevan