Move him to a cooler place immediately and offer some water at room temperature, not cold. Place cool damp flannels across his ears, forehead and legs and mist his ears with cool water.
Here's some ways to prevent a heat stroke in the future:
Provide an area for him that is out of direct sunlight. A little shade can make a big difference in temperature.
Stone or ceramic tiles will provide a nice cool feeling on their bunny bellies.
Air conditioning can help alleviate those warm feelings.
Open windows that provide a breeze.
A fan can also be used, but do not have it blow directly onto your bun.
Groom your rabbit. Helping your rabbit to rid themselves of excess fur will help cool them down.
Freeze some water bottles and leave them out for your bun. They enjoy laying in the cooled air around these bottles and may even lick some condensation off the sides.
Water! Provide plenty of cool fresh water for him. You can add an ice cube or two and watch bunny happily lick at them.
Give em a spray. Rabbits use their ears to regulate temperature, so by spraying some water mist on their ears you can help cool them down.
Never get the ears completely wet; a quick mist will do.
A cool damp towel draped over an area where they hang out, their cage perhaps. Make sure it is not dripping right on your rabbit
Veggies! Give him vegetables to help keep him hydrated. You can leave a little water on them after you rinse them off to add to their water intake.
