Okay, so you've provided water, a cool place, shade and fortified your dog's diet against heat stress, exhaustion or heat stroke and still your Boxer is in distress. What to do?
First, call your vet and tell them what is happening. Follow whatever directions they give you. If you cannot reach a vet, do everything you can to get your Boxer's temperature down. If you can, put your Boxer in a bathtub of cool water. In the meantime, splash water on the bottoms of their paws, tummies, private parts and ears.
If your boxer is conscious or still responding, give them water to drink. If they are panting too hard to drink the water, trickle just a little in their mouths (not a lot or they might choke).
You may place ice packs on their chest and underbody areas. Keep trying to contact your vet in the meantime. Depending on how severe the heat stress/echaustion is, your Boxer may be in shock and require immediate medical attention.