aaaw poor, poor little thing. where to start?

first, get him eating repashy crested gecko mrp immediately. clark's is merely filler and will not sustain this gecko with the nutrients it needs to survive. add a small amount of calcium to the diet for now to counteract mbd which looks quite probable. feed him 3-4 times per day and leave a shallow dish in his enclosure as well.

you may want to get some pure calcionate syrup (calcium glubionate) from a pharmacy to give him as well (this would replace the calcium added to his food)--the dosage for this is 3 drops per 100 grams so you will have to be careful about the dosage, but the good thing with it is that it is pure calcium and geckos will not overdose on it like they can with calcium that had d3 added.

what kind of antibiotics did you give him and how much? he needs to see a vet asap for his mouth which looks like a combination of mouth rot, mbd and injury. in the mean time you should clean the area with chlorhexadine or betadine.

you should mist and give water by hand right now. let him have as much as he will drink every few hours until he gets over the initial dehydration.

he is probably opening his mouth because he is stressed but he could also have a respiratory infection. have the vet check for this when you take him to have his mouth looked at.