The 18-year-old is likely to be on the bench for Tuesday night's clash with Preston after experienced keeper Michel Kuipers left the club last week.
Manager Richie Barker told the club website: "We are delighted to have Johnny on board. He has been training for a few days with us and you can see that he has all the attributes to develop into a good goalkeeper.
"He is working with Sunderland's first-team squad and has been a regular in their Academy team. This will give him more experience and, of course, provide us with cover for Paul Jones."