[moondog]

25 celsius isn't that warm, however when the temps warm up both my dog and I go through a period of acclimating to the higher temps. Humidity will play into it, also....the higher the humidity, the harder the body has to work to regulate internal temps.

Since dogs only "sweat" through their pads, they have to resort to panting to help regulate their body temp, so it's more likely this is what you are seeing, rather than him being "winded". Heatstroke is a big concern with dogs. When your dog "quits", he's telling you what his limits are. If this point is out of character for him without an obvious environmental cause (temp and humidity), I'd say a vet check is in order.