Not sure which radiation treatment you are having but mine was using the Mridian viewray machine. They were able to see in real time how full my bladder was and how it was changing as I lay on the treatment table. After the first treatment with 50 oz of water 50-60 minutes before my treatment, they saw that my bladder could be uncomfortably full and from then on 30 oz did the trick for the remaining 4 treatments. Maybe testing the amount your husband can hold outside the medical facility for a time your RO can suggest might help. Good luck. They are trying to limit healthy tissue impact in the "bladder" area as different radiation machines have different margins.