The 30-year-old star and his actress wife Elizabeth Chambers are already parents to daughter Harper.
The couple intended to keep the gender under wraps but Hammer hinted while talking to Kimmel on "Jimmy Kimmel Live!" that he would no longer be outnumbered at home.
"You can find out now at 9 weeks, which is really bizarre. They do a blood test. It's crazy. They test if there's hormones in there.
"They go, 'Oh, you've got testosterone. So you're having a baby boy."
"We're thinking Jack, Jack Hammer. Michael Charles and he can just go by MC Suddenly, he realised his mistake. I think I just gave away our baby name. I'm in so much trouble.