Cute little guy, it's good that you came her to get advice on his/her care. What is his name [ sorry if I missed it ] He's about the size of a 2-3 month old, possibly 4 but since the pet store or whoever sold you the lights gave you the incorrect stuff , that is most likely what caused the lethargy + stunted growth. Hopefully with the proper lights he will start doing better but it may take a while. Drip some water on his nose for extra hydration if he'll take it.
It's also possible that he has parasites so if his appetite doesn't improve in a week or so you can get a fecal test done. You mentioned that a vet had given him meds., what were they ?