It was finally revealed in a People Magazine article on Tuesday May 31st, that Jackson Roloff, Zach and Tori's son, does indeed have achondroplasia dwarfism (the same as Zach).
Fans have been speculating about whether Jackson has dwarfism or not. The Roloffs kept it a secret until the People Magazine article. They also were secretive with pictures of the baby (some Roloffs posted and swiftly removed pictures featuring Jackson) until the People Magazine article was published. It is common for celebrities and people on Reality TV Shows to sell the "Exclusive" First Pictures of babies, weddings, etc.
Zach tells People Magazine:
“You have to encourage a dwarf child a little more because it will take them five steps to do what others can do in two,” says Roloff. “But I knew, dwarf or not, I was going to parent my child with the mentality that not everyone gets a trophy. You have to earn it.”
“The world is changing. People are more open to diversity,” says the reality star and soccer coach, who hopes nothing will impede Jackson from achieving his dreams. “Whatever he wants to do, we’re going to find a way to help him do it. That’s our job in life now.”
The full People Magazine article is Here