Lily,
It took my husband almost 2 months after his last radiation session to feel even close to normal again. if your husband is on Fentanyl, that could also contribute to his mood: anxiety, irritability, and depression are all major side effects. If he has just finished treatment, he may be angry that he doesn't feel better yet. My husband thaought he would feel better in a couple of weeks, rather than months.
If your husband follows the same recovery pattern, he should feel like himself again by the end of the third month after treatment.
It was so hard to remind myself that it was the drugs and the pain and not my husband talking when he snapped at me. he told me he was sorry when he felt better - about 2 months post treatment.
Hang in there. It does get better.