He's going through his "obnoxious preteen" stage, perhaps. Obedience training NOW! and let your instructor know you are concerned with his aggression, and need to nip it in the bud right now!

Is he aggressive with you, or just with other dogs?

Bless you for taking him in! Hounds in general can be on the stubborn side - so much so that some people think they are stupid - far from it! They're often just smart enough to know what they can get away with!