how often and how large are your water changes and is he more likely to be like this after changing water ?
ive seen rays do this after water problems usually after large water changes. if this is the case then try small more frequent changes. if you can monitor temperature and PH fluctuations before and after water changes to see if they are changing too much causing the ray to be stressed.
is the tank well filtered ? if not this could also cause water quality issues causing the ray to be stressed.
rays can often look like they are eating when they are just "mouthing" the food but not actually eating it especially if the food is on the larger size. only way to tell is if the food is actually disapearing which can be difficult with other fish in the tank and alot of substrate but not impossible.
failing that, could it be the aro picking on the ray ?. stressing the ray out and causing the ray to behave like this.