What Causes Legs Pain After Quitting Heavy Opiate Addiction?

My husband is quitting a pretty heavy opiate addiction, but it has been about 25 days and he is still in pain in his legs to the point that he doesn't want to get out of bed. I know withdrawls are hard as I quit at the same time, I just don't understand what is taking so long for him? I am okay. How long do you think he should feel like this? Should he go to the doctors to make sure it is not something else wrong? What could it be?