if his movements are really jerky, like he is trembling, sounds like he is suffering from calcium deficiency or other mineral deficiency. as you didn't explain any thing about his setup or feeding regiment, hard to say where it is lacking or needs improvement. are you feeding him baby food and insects, just babyfood fruit and meats? or are you using the crested gecko diet powder by t-rex?
if using any of the babyfood methods, add a pinch of calcium powder and multi-vitamin to the babyfood before serving it to him. if you are already adding regular suppliments or using the crested gecko diet powder, something else could becausing the shaking.
if the gecko has been exposed to toxic substances, either wild caught insects who have ingested insecticides, any insecticides or poisonous substances used in the house recently [ie painting, urathaning/varnishing woods, mite/flea/tick sprays etc] could have poisoned your gecko.
my suggesting, if your diet and temperatures are optimal take your gecko to a herp vet in your area. something else could be going on that isn't readily apparent. there are some diseases that can affect crested geckos though i ca n't remember off hand what that disease is but i do beleive one of it's symtoms is shaking or sudden loss of weight.