If Holder brings charges against him, he will lose benefits and pension too (dishonorable discharge). This is the stick the president holds regardless of what congress can say about the investigation. I don't know that congress can grant him immunity from Holder. Let me know if I am wrong.
The President can still threaten him into silence about Bengazi.
“If Holder brings charges against him”
What charges? They would have to charges of a high nature to lose your benefits, such as willingly divulging classified material.
Just having an affair might result in a reduction of rank, but it would take a conviction resulting in prison time to lose all benefits.
Remember you are talking about a 4 star general.