First time raising a caterpillar..

I am not really sure what is wrong with your larva. In the picture his color is good and he looks very fat. These are good signs of very healthy larva. Some larva can be sick with bacteria or fungus, have parasites or genetic faults that can cause problems you described. It surprises me that your little larva suddenly became ill if kept in a very clean cage. So I believe he may have a wasp or fly parasite inside him. But on the other hand larva can suddenly become ill from ingesting bacteria or fungal spores on leaves. I am rearing about 200 Buckeye larva from eggs and have lost only about 12 to such leaf born deceases. They ether suddenly stop moving and die or they spit up fluid from the gut then die. I do not disinfect my leaves in water with a little bleach like some folks do because I think this eating of spores by the surviving larva makes them stronger and healthier.