I was very tired on it as well.
Talk to his doctor about it and see if they can increase some vitamins that get depleated while taking the med. I know they perscribed something for me on top of having me take some OTC vitamins. Not sure if there's a difference between men and women and what they need to take.
Also, have him make sure he drinks a lot of water daily. It supposedly helps. And, if he takes the med 1x a week, I know some people take it on days (or was it the day before?) they can plan to take it easy and rest as taking the dosage wipes them out.