Is this normal with APBT pups? He will never pant like my GSD, he just keeps his little mouth closed. :)

;)

Could be an asset when he gets older then ;-)

How are the dogs getting along now?.
Part of the breeding of APBTs is heat dissipation, and Stamina, no shortage of breath, etc. With normal dog play I would expect the GSD to be worn out a long time before the APBT. The APBT is a pup I would expect when he reaches the 12 to 18 month stage you will be looking at a couple of hours of serious exercise to get him breathing heavy.
Can I suggest you buy a book on APBTs and learn a bit about them. You will be rewarded with a great dog. People who have owned an APBT for its lifetime will tell you that its the only type of dog they would want to have. Sadly in the uk this is not possible.
Mick

Thank you Mr. Murphy,

I have obtained a book, I had one before I posted this. It never addressed the issue specifically. The dogs are getting along pretty well, although sometimes I dislike how rough they play, they never seem to hurt one another. Also, it seems that at his age (10 weeks) he is a very mellow dog compared to the GSD. He just likes to sleep.