You can do partial (20%) water changes, each time using water that's a couple of degrees cooler than his tank water - not too much cooler, as you don't want to shock him by changing it too dramatically, but remember when you're putting in only 20%, it's going to dilute the temperature change accordingly, so you can use water that's maybe 6 or even 8 degrees celcius cooler for that 20% you are putting into the tank. Then, use a fan directed across the top of the tank to keep it cool. You can also put the tank down onto the floor, which is cooler, or into a cooler part of the house (basements are always cooler than attics). You could float a plastic bottle with frozen dechlorinated water in it in the tank, but you'll need to keep replacing it with another one when it's about two-thirds melted (otherwise, there will be continual temperature fluctuations, which are stressful), so this is more of an emergency measure than a way to keep the temperature down permanently.
If you can afford one, you could invest in a chiller - depends how hot your summers get and whether you can get your temperatures down in other ways. Aim for 16 to 18 degrees celcius. It should never get above 20.