It looks like there might be a teeny bit of something else in there, but it's hard to say... I thought pitt too, then I also thought that would likely make him shorter, but a little something in some of the facial shots gives hints of it to me. Maybe mastiff but I'm not as familiar with those, haven't met very many in person, so ???
But who the heck cares? If I saw him coming down the street I wouldn't say "oh there goes a 95% boxer" I'd say "There's a boxer! Let's go meet the boxer!!"
And what really matters is you rescued him, which makes you an angel, and automatically makes him part of the boxer family here.
He looks like a loving, wonderful, fun dog... all boxer traits... and he's handsome too!