Hi there and welcome to the forum.
Fish are cold blooded animals, and so their bodies are the same temperature as their surroundings. 15C is a very low temperature for a fish that needs around 27C. I don't think pouring in hot water was a good idea, even if you feel like you went slow with it, it may have been too much change for his little system to handle.
At this point, there really isn't much you can do. Change out 50% of the water, lower the water level so he can reach the surface more easily, and put a towel over the bowl so it's dark (to reduce stress).
Only time will tell if he will be able to pull through.