If you haven't already have his thyroid checked. Have them run a TSH, T3, and T4. Quite a few on the forum have had thyroid issues due to the meds. And definitely the water.
I had a problem with the exact same issues you described with your husband both during and after treatment. I tried taking in more fiber with Citracell which only gave me 2 grams of fiber per serving and eating more nuts and veggies which helped a little. But I felt like I was having to eat a ridiculous amount of fiber-rich foods to get up to 30-35 grams a day of fiber.
Last week I started drinking a protein shake 2x a day that has 8 grams of fiber per serving in an effort to lose some of the 15 pounds I gained post treatment and to boost my fiber intake. I lost 18 lbs. during treatment, and post treatment went hog wild with food when my taste buds started working again and gained 15 lbs. of fat. Since I started taking in more fiber I've shed 5 of those lbs. and my constipation and hard as rock stools are gone and everything's back to normal.
Sounds like your husband could be dehydrated. Lactulose can cause dehydration at the same time. Have you tried dealing with this with fiber and drinking recommended daily water intake. If I don't drink enough water everyday it leads to constipation. I really noticed this after treatment. It's like now I require even more water. Your body is also trying to get rid of all the
medication taken on treatment.
DWBH