One reason an undertank heater will be better than another light bulb is that leopard geckos digest their food much better when they can lay on a warm surface. Temperatures should be (as posted above) 88-90 on the warm end and about 80 on the cooler end. Those temperatures should be measured at floor level, not a few inches above the bottom.
It could just be the angle of the picture, but if any of his feet seem to turn in at the wrist (or look bulgy like Popeye's forearm), it could be a calcium deficiency even though you have calcium available in the cage. This year I found out the hard way that some young leopard geckos will not go out of their way to lap up calcium as the adults will. I've been using a liquid calcium supplement with Vitamin D3 to help the leo(s) absorb it better. For hatchlings and juveniles I'd also make a point of dusting their food with calcium powder at least once a week (in addition to a calcium dish).
Once your temperatures are fixed, he will probably be eating 8-10 mealworms an evening.
Good luck with him, you're off to a good start. Post back if you have any other questions!
>>okay, so Freddie has a 25wt. blacklight and the air temp is still only 80f and he hasn't grown much bigger scince i got him. but his tail has so are the temps wrong?? The pictures might not be here for a while. sorry about that. thanks for your help P.S. he is about 5-7 inches long and eats 3-5 mealies every other day. and has a bowl of calcium in his cage.
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>> thanks you for you're help
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Andrea A. 