Poor little guy, I wonder how it split open like that ? His arm may or may not be broken but it is an open wound that hurts , so he would be favoring it. Can you get to an experienced reptile vet ? You really should try to get him to one. If you can absolutely not get to one, buy some raw, unpasteurized honey to put on it. The kind from a health store, very few grocery stores carry raw honey....it must say raw + unfiltered or unpasteurized ,it's a natural antimicrobial. Add a tiny bit of warm water to thin it. Apply to the wound with a Q-tip and cover with a bandaid . If the bandaid is large, cut it to fit. Then check it every few days + change the bandaid, but be very gentle. If you can't get the bandaid on, just apply the honey 2X a day. Keep any loose insects out of the tank [ feed him by hand or just be sure non are loose after feeding ] Give him water on his nose to lick via syringe or eyedropper to keep him
hydrated.