Do you remember that show with the boy?

I'm trying to figure out the name of an old cartoon i used to watch. It was about a boy who was really smart and he solved government issues with the help of a big huge bodyguard who was his partner. I remember one of the episodes where a train crashes into the ground breaking tubes with a type of gas that kills people by making them see there worst nightmares. While the boy and Big guy try to solve the problem the big guy gets infected and so does the boy. but the boy uses a yo-yo to defeat the bad guy supplying the fear gas. and they find the antidote that saves the big guy. any of this wringing a bell to anyone?