There's so many factors that could be affecting your husband's behavior. Don't know what chemo he's had, but my chemo regimen usually caused a 'nadir' about 10-14 days after each session when my energy was lowest. Radiation can also cause fatigue, although I was fortunate that the treatments I got caused only mild effects. Dexamethasone, especially in high doses, is a vicious drug - I had major mood swings, paranoia, insomnia, shaking, and weight gain related to dex. Tumours also cause all sorts of behavioral issues.
For me, an antidepressant helped with some of my issues, you might want to suggest your husband talk with a psychiatrist. Cancer, chemo, radiation and dex are all very stressful alone, in combination they can be overwhelming.