Here's a couple of my thoughts, there could be a couple of things going on that I have seen. When you say you are feeding him shrimp, is it table shrimp, or krill. It is not unusual for an antennata to just stop eating dead food after a while. If that's what it is you may try and introduce live food again, sometimes it works, sometimes not, most times not. If you have been feeding krill exclusively all this time, it could be lockjaw, which I don;t know how to reverse. Feeding one thing exclusively can also lead to lack of complete nutrition and lead to lockjaw. Lions will sometimes also just go off eating from time to time, does his color look pale or even somewhat cloudy eyes, he could be getting ready to shed his cuticle layer. This usually would only last a week or two though. I would throw in some ghosties to see if that interested him at all. Sometimes a lion that came from a source that uses copper in their system, or if they have been treated with copper, or have had repeated treatments of antibiotics or other meds; will just stop eating after several months. The med exposure and lockjaw has no treated that I am aware of.