J.S.
Why haven't you just removed all dairy from his diet? You seem so insistent on giving him lactose products but they're making him sick. The solution is not to treat the constipation but to eliminate the lactose completely and forever. I realize that it seems inconvenient, especially if no one else in the house is lactose intolerant, but there are substitutions that are safe and lactose free. It's a lot easier to transition to a lactose-free diet for him than it is to put him on needless medication and have him in pain, sick all the time.
It will take two solid weeks for dairy to leave his system. It will take two more weeks for his body to adjust and feel good without dairy there and for him to realize that "Hey, this is worth not having that glass of milk and grilled cheese sandwich! I miss pizza but not if I'm going to be so sick!"
And you may have to search for lactose-free chocolate and other substitutions, but it's worth it or else you can cause some serious damage to his insides.
I'm a mom to a child with severe lactose intolerance. I know what I'm talking about.