Also, if you cant get him to eat veggies, i've read in lots of places that if you soak the food in garlic, and/or push his normal ration (suck as algae wafers or shrimp pellets) into the veggie, they will be more likely to explore it. This hasn't entirely worked on mine yet, but theyre stubborn, and seem to prefer meat over veggies.
However, if he keeps playing, and his tummy isnt concave (if it IS concave, it would mean that hes starving), and he doesn't lose weight, he may just be enjoying playing.