-Es-tu fier de lui? (are you proud of him?) -Un peu que je le suis ! (Of course I am !) Frankly? Better stick with common french, you can simply say "Bien sûr que/oui je le suis !"

Yes it's a slang. No it's not commonly used. We could translate that by "Bien sur que je le suis" Ex: "Tu es sportif?" -" un peu que je le suis!" --> "Bien-sur que je suis sportif!" I hope I could help you!

I love this expresion! You shouldn't think of using it or not according to what Grammar says, but according to what French people say and in which context. This expression is totally correct if you're in an informal context. You can hear it from people who speak with you, in movies, etc. So it's good to know it. Of course, don't use it in a formal context or in an exam!