Actually he might be fine taking extra magnesium pills to ease his constipation, if he takes them the right way.
I'm on a medication that causes severe constipation. I could eat masses of fiber and it would do almost nothing. Lots of liquids, prunes, etc. Laxatives gave me horrible cramps, plus you can't use them often.
Before bed I take a calcium/magnesium supplement for sleep. Once I accidently took them twice. No constipation the next day! And no cramps at all.
Turns out one of the reasons many are deficient in Magnesium is that the intestines don't absorb it well. Taking magnesium supplements to correct the deficiency often doesn't work. You end up with diarrhea because you took more at one time than can be absorbed. All that extra magnesium just gets flushed out of your system and down the toilet.
You can deliberately take more than you can absorb as a laxative. From what I was told, you can't be poisoned by this because the whole point is you are NOT absorbing it. There is a chance, esp. if it causes diarrhea, it may lessen the absorption of other medicine, so take them at different times. And you do need to keep well hydrated, since the magnesium uses extra fluids to loosen things up.
So I bought the cheapest magnesium supplement I could find (the cheap ones are the least absorbable) and take it before bed every day. If I miss a day, back to constipation city. It took some experimenting to find out how much to take. It's around 1000 mg for me. I do have to vary the dose a bit. For example, if I ate a huge bowl of chili or black bean soup, I'll take a bit less.
This is just my own personal experience, but it's worked great for me for years. Good luck finding something that will help your husband.