How do you increase pubic hair?

Basically, you don't. The amount of pubic hair that a boy has varies from person to person. Your age and the stage of puberty you are at will also have a significant effect, as will genetics (amount of body hair is likely to be an inherited characteristic). Essentially some people have more hair on their bodies than others. Next time you are at a beach or swimming baths you might notice that although some men have hairy chest others don't. Some may have shaved their chest but many just won't have grown any hair there at all. It is the same with pubic hair. Most men will have it but some wlll have more than others.