It sounds to me like a possible heat stroke.
Did this happen today?
The heat of the day has passed, but you may want to mist his ears with water, or hold his ears wrapped around ice cubes to make sure he is sufficiently cooled down.
You must get fluids into him. If he won't drink water, perhaps he will drink tea, diluted 1:1 with water. Or Pedialyte in some fruity flavor. Even Gatorade. Or water with a little molasses mixed in. If he will not drink, you can try syringing any of the fluids above into the back of his mouth, from the side. If you have no syringe, maybe you could use an eyedropper.
If you cannot get water down him this way, you can inject a subcutaneous bolus of water. If you happen to have an unopened syringe and needle, and an unopened bottle of contacts saline solution, you can do this tonight.
You can make your own saline, but in order for it to be sterile, you must follow proper sterilization procedures:
http://www.ehow.com/how_7636306_make-ow ... ution.html
I do not know how much you would give a rabbit, and I cannot guarantee that anything will work. Your bunny is in an emergency situation, and I'm giving advice that might help, but I don't know.
Here's a page on giving such a bolus to a dog:
http://www.vetmed.wsu.edu/cliented/dog_fluids.aspx
Make sure, if you try that, that you do not begin injecting before you first pull back on the plunger (which means you have to leave room in the syringe to do so). If you get air or blood, you must remove the needle and try again. You would give it wherever you could get enough skin together to give it. Sometimes it's the front shoulder area, sometimes it's near the back leg.
Hopefully, you can syringe fluids into the bunny. That would be much easier, I'm sure.
If he does not improve with fluids, it's probably time to put him down.
Whether he needs to be put down now is entirely up to you. No one here would fault you.
Please keep us updated. :clover:
