Dislocated Arm?

Hi, 3 days ago my 3 year old axolotl was not moving as much as he usually does and was floating on one side. He isn't bloated or ingested any small stones (river stones on bottom of tank). Upon closer inspection we noticed he was holding his left arm close to his body and not using it. I've researched on these forums and google for something similar and was only able to find one topic that didn't really help.
The floating on his side seems to be compensating for him not being able to use his arm. At first I thought he was faking it (he is a weird axolotl so wouldn't be surprised if he was) because he was using his tones to grip onto things but not moving the whole arm.
Has anyone else had this happen or is there something I should do?
He is eating normally and doesn't seem too stressed out by it - so should I as his mummy be worried?

Thanks!

He could've gotten his arm stuck inbetween those rocks. If he's eating fine then just let him heal and try to not stress him out. They aren't the brightest of species.