First of all I agree with others with the fact that you're just trying to be a good parent to your baby! But it does sound like just general aging symptoms, all things I went through with my last dog, who was 17 years old (not a boxer). How my vet put it to me when I was starting to get really worried about her as she aged was, "you and I may not be able to keep the pounds off or be as active as we once were either, but that's no reason to put her down yet." And the signs of her aging started when she was 10 years old, and look she went another 7 years before I saw the quality of her life being to difficult on her. Just so you know, as they do age there are meds out there to really help them stay comfortable, we went from using buffered aspirin as needed for her old aches and pains, to having to increase to a mild narcotic drug to keep her more comfortable on her bad days, to doing a steroid injection (which really brought the life and energy back to her for a little while).
As for the Cushings, I can tell you it is hard to differ between that and the later stages of the aging process, as my girl aged alot of what you see in Cushings does look very similiar to a very old dog. However one distinct thing to notice with Cushings, or Diabetes which can occur as they age also, is any abnormal, persistent smell from the dog. With Diabetes they will increase their drinking and urinating and have a fruity smell to their breath. With Cushings the dog might start smelling "stinky" all the time no matter how much you give them a bath. They have that "wet dog, old gym sock" smell and start shedding more. This was the sign my sister's dog had when they took him to the vet and found out he has Cushings. They only did the urine test that the other person posted about because he's an older Rottweiler and has surpassed his life expectancy anyways my sister didn't want to spend the money on the more expensive tests since the urine test was pointing towards the Cushings diagnosis, and the meds for the Cushings doesn't treat it, it only extends their expectancy and they are very expensive meds. So they just keep him as comfortable as they can in the mean time.
Sorry this post ended up being so long, but hope that last little part of the smells to watch for helped.