What happens if he holds him in contempt of Congress.
Thnx
It gets pretty complicated. Depending on what the contempt of Congress is, Congress has a number of options available to it. It can pass a resolution condemning the contempt (just words). Or, it can pursue a civil action in court - more than words. And, if it rises to criminal contempt - like you've lied to Congress while under oath, then Congress can formally request that the AG pursue a criminal contempt case, which has happened about 10-or so times in US history, OR there's even some case law that suggests Congress can act independently of the AG, and have the Congressional Sergeants-at-Arms effect arrest on the US official. This has not been done since the 1920s or 1930s.
Usually, when the Executive Branch starts ignoring Congressional subpoenas, whispers of "Constitutional Crisis" start soon thereafter.