His contract, as I understand it, pays him half a million per game and the rest is incentives. If he doesn't play, he doesn't get paid and he won't reach any incentives. So yes, NE would have to honor his contract, to the extent that it obligates them to anything. But as a practical matter they won't have to pay him anything other than what they agreed upon as a signing bonus.
He would become a free agent in 2020.
According to Spotrac, this is what they have listed for Brown's contract . . .
Antonio Brown signed a 1 year, $10,500,000 contract with the New England Patriots, including a $9,000,000 signing bonus, $10,000,000 guaranteed, and an average annual salary of $10,500,000. In 2019, Brown will earn a base salary of $1,000,000, a signing bonus of $9,000,000 and a roster bonus of $468,750, while carrying a cap hit of $5,968,750 and a dead cap value of $10,000,000.
I believe that close to half a million dollars you mentioned is not per game . . . it's over the course of the season.