He will given immunity and then required to testify as to his sources. When he refuses, he’ll be in contempt and will be locked up until he relents. Which would leave this in President Trump’s hands.
And that will kill WikiLeaks. So he won’t do it. He wants to testify about the reasons for government to function openly.
If the government insists on making it an investigation into his sources, he wont even show up. That is diametrically opposed to his purpose in life.
If that’s the case that they demand to make it a source hunt, he will stay there until he finds a way out either through escape, or through the British government backing down. Then he will find his way to safety away from the US Gov.