No do not put any other turtles in there if they are close in size they will out compete it for food. Plus when they get close in size the may become food for the snapper. How deep is the water he should have a basking spot that he can climb out on completely. Try live fish for awhile also if you are living in a cool area it may be preparing for brumation(hibernation). generaly though if you place the food near it ie. meat or frozen bloodworms it will take a few bites of it. They usually don't eat pellets while young or any age really at least the ones I've seen/worked with.