If he develops gas from not eating properly, follow the suggestions below. Your bunny is sitting hunched and still. In some instances, he may stretch out fully, giving the appearance that he is trying to press his belly to the floor. You offer him treats, and he is totally uninterested. In fact, his gut sounds, heard through a stethoscope, can be deafening.
Heating pads can cause severe burns and injury when misused.
Some vets, who see these injuries frequently, discourage the use of heating pads altogether for this reason. Do not use any setting other than low for your rabbit. Take his temperature periodically to monitor the effects of your efforts to warm him. Whatever else you do, fuss over him. You can warm the sub-q fluids by putting the bag into a pan of water, bringing the water to a boil, and cooking the fluid bag for several minutes.