at 1.5 inches hes most likely around 4 months old. Im sure that the one you got orignated from a batch of eggs from a tonmo member here named Righty. I have 5 brothers or sisters of your cuttle here as well..=)
At this age he should be able to instantly eat anything that falls into the tank. They are quick varocious eaters. Brine shrimp is actually not a great idea for food for they lack nutritional value and are pretty small for your cuttle. Individual personality will vary, but it looks like yours is shy from the move. If you havent actually seen him eat, and its been more then a few days, I would think about keeping him temporarily in a seperator. The bigger you can find the better for 1.5 inches is a little big to hold him in a breeder. I would also double check your water parameters, because they will start losing interest in food if the quality is not up to par. Also, ensure your salinity is 1.025-1.026 and temp 72-80 F. The best food to feed them is marine shrimp or marine crabs. Fresh water foods lack all the nutrional value they fully need, but can be used intermitently. Yours should be able to eat guppies, ghost shrimps, marine shrimps and small crabs (remove the pinchers to be safe). He would eat pods and brine, but its more appropriate to feed him something bigger. I would forget about the brine shrimp all together actually..
How many days has it been since you acclimated him?