Dog breathing somewhat excessively after exercise

I am keeping a dog for my daughter he apparently adopted it at college, right before moving into sorority house. (If you lot of pics with you child walking friends/neighbors dog at school beware. ) I didn’t take it immediately to the pound because she was attached to it and she got it for protection and there was a series of rapes and sexual assault close by so I certainly understand.

The dog is an Australian shepherd/pitbull mix. It’s incredibly bright and overall great dog, although it’s been around people so much sense to have a little bit of anxiety issues if left alone. It looks scary but has none of the aggressive nature of some pit bulls.

I have noticed over the last couple weeks that he seems not to be able to run and catch the ball as many times as he has previously, and seems to breathe a little bit harder and longer after exercise, but otherwise is in perfect health. When I was growing up, we always watch out for symptoms like this for potential signs of heartworm infection but we’ve got him all the medication that I’m bringing him to be checked tomorrow just to be sure. Maybe just allergies or heat or something?

My vet is notorious for wanting to run thousand dollars a test for everything, which I definitely want to avoid. Should he be checked for anything else ?