Hi there, this is Dr. Elaine.
"Blockage" can be either within the gastrointestinal tract (i.e. he swallows a foreign object that gets stuck) OR within his urinary tract--i.e. he cannot urinate.
I would be very concerned about the latter, and given the very RAPID onset of his symptoms, he needs to be seen on an emergency basis ASAP.
An arched back is indicative of pain--could be abdominal (i.e. bladder, stomach, intestines)--spinal (back) pain can also present these symptoms. My greatest concern would be for a cat experiencing acute abdominal pain, which can be life threatening, especially if he is blocked (i.e. cannot pee).