Locate frozen mysis shrimp, formula 1, frozen plankton, frozen cyclopezee. My best suggestion here is to thaw out a cube of one of the above rinse in RO water then some tank water and serve.
You may have to try more than one or let him go hungry for a while till he bites. You might also try a drop or two of garlic to stimulate appitite.
If you continue to feed brine then I suggest that you find a vitamin suppliment to soak the food in like seclon, zoe, or zoecon.
Thomas