As a young activist, Corbyn opposed the Maastricht Treaty. As an MP he voted against the Lisbon Treaty, so his anti EU credentials are firmly established.
The 2016 referendum was a free vote, and many Labour MPs, Councillors, members and supporters voted to leave. So in all probability, Corbyn also voted to leave.
Not even sure why this is open for discussion. A more pertinent qustion would be how a Prime Minister who has been a lifelong supporter of the EU, has been allowed to make such a mess of us brexitting.