He voted Leave in 1975. I think officially he's said he voted Remain in 2016, but as ich says there are rumours to the contrary and he has been very eurosceptic throughout his career. He was obviously completely uninterested in campaigning for Remain, let alone voting for it.
Given that his core supporters in the party are overwhelmingly pro-Remain and that he also needs to somehow keep the support of Brexity Labour voters at the same time, I imagine for the moment he's happy to let both sides think he actually voted 'their' way.
Still, as others have said, his political instincts are fairly obviously in favour of Leave. Which means he is in all likelihood the most powerful Brexiter in parliament.