If your dragon is in that bad a condition, need to know background on him. Was he a rescue you acquired? Or did you buy him and he started to decline? Feeding by syringe is good and one way to get food back into him, but if he became in this condition due to something missing in his setup, the only way to ensure he has the best chance to recover is to correct the error in his husbandry.
If you can give us a detailed account of how you are keeping him, if he is a rescue (either given to you by friend etc, or 'rescued' from a petstore) then it is just a matter of setting up the ideal setup for him, free of distractions in a quiet area of the house so he can concentrate on getting better.
Another good thing to add to the 'insect shake' would be juvenile bearded dragon pellets. These will help balance out the liquid diet and in a way, much easier to turn into a liquid than crickets etc. Oh and silkworms/butterworms would make a better choice in an insect shake than crickets, mealworms or superworms. Less exoskeleton to gum up your blender. Silkworms liquify very easily in a blender...I known I tried it, with the thought of freezing it and using it as a nice treat for my geckos....ground up silkworms have alot of water and doesn't smell very nice. It is also an icky black color LOL.
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PHLdyPayne