Lovely dog!
Pits and bulls are know for their farthyness if they eat meaty foods!
But if he's underweight you should try and feed him one meaty meal a day besides his dry food.
My Bull terrier loves the Cheap cans of Lamb and rice or beef, but he doesn't get that all the time, cuz he smells so bad then...
Nice colour on your dog, same as mine!
Keep us posted with some pics of the stunts he's gonna pull, cuz I know he is going to pull some!

Any idea how old he is?
And is he listening a bit?
Mine never does...but is very sweet.