Did you match the sg before releasing him into the display, any sg higher than .001 difference would need an extended acclimation to match before release. The stress of the move could have triggered liver failure due to copper poisoning. If your lfs runs copper in their systems, this is a very common occurrence. The stress of the move triggers any underlying condition, if his color was all washed out when you found him, he could have seized likely due to cyanide poison. If he didn't have any wounds then I would rule out a bacterial infection. He would not have interest in food if internal parasites where advanced enough to kill him. These are some of things I've seen when a lion dies within a few days and the stress of the move is what triggers it. When you say he was eating frozen, were you target feeding him chunky bits of food, or broadcasting the tank with food like mysis. If he was eating frozen at the lfs by broadcast methods, it could be a high chance that he just starved. Almost exclusively dwarf lions need live food like ghosties or guppies to settle them in, then need to be target fed chunky bits of food, not expected to sustain from broadcast feeding. They rarely live much more than a year when people are determined to feed them a dead only diet, the dead diet most people feed rarely gives them the nutritional needs necessary for long term success.