If my dog still wants to run and play in the heat surely he’s fine?
Depending on how hot it is and the level of activity this may not be a good idea. Instead, to be safe it’s best to exercise your dog in the early morning or late afternoon/evening if you know it’s going to be a hot day. If you have no option but to exercise your dog on a hot day take frequent breaks, offer plenty of water (at least as much as you are drinking), exercise in the shade, preferably near a water source such as a stream, and keep your dog’s coat damp.