Guide him. There's not much you can do about the impatience he inherited from your father-in-law, but you can help him learn to calm down and cope; teach him not to yell by modeling quiet ways to deal with frustration.
Adjust to him. Surprised that he doesn't like crowds or noise when you're a social butterfly? You're probably not going to succeed in changing your adorable wallflower by dragging him to lots of parties. Instead, try keeping his environment predictable and peaceful. Then, eventually, you can introduce him to your world.
Celebrate him. Even if he's not the party animal that you are, find the positive in his cozy, stay-at-home side. Your acceptance of him will have a lasting impact that can outweigh his genetic legacy.