iirc he's still going to the convention. maybe he's riding on the small chance that hil will actually face charges and then get the nomination by default
He promised he'd go to the convention, so he's keeping his promise. He's also helping push for a more progressive democratic agenda during the convention.
And now that I've had more time to think about this, this endorsement actually puts a lot of pressure on Hillary. When she inevitably breaks every single one of the promises she's made, Bernie can leverage that against her and other establishment politicians in order to make himself more favorable and them less favorable.
On top of that, he's having over 100 progressive candidates running for local elections now. This, my friends, is how a revolution starts.