thats a nice strong looking dog you have its not your fault the dog has mange!!.
i bought a bullx aswell and at the time she didnt look to heathly and was very quiet and nervous looking but nothing a bit of food/tlc etc wouldnt fix now she's bouncing like a diferent dog altogther all the best with him hope he does his job for you
i would love to see another photo when his skin clears up