First of all, my.dad.is.sick, I want to welcome you to the forum. You will find a lot of information and support here.
I've never had the "joy" of having to take Lactulose, but from what I've seen posted here, your dad SHOULD take the amount prescribed. Doctors like to see 2-4 bowel movements per day, as that is how the excess ammonia in his body is eliminated. If he doesn't get rid of it, it will go to his brain (hepatic encephalopathy) and cause many symptoms which are a lot worse than having gas, diarrhea, what-have-you. Sometimes the amount has to be adjusted up or down.
What type of liver disease does your dad have? When was he diagnosed? He is a candidate for liver transplant?
Connie