Eyes look clear, color is fine, and fins are intact; he is a little but otherwise looks good. If he's been at the lfs for 4 weeks and was re-homed, he's likely free of any treatable issues.
What was/is he eating; I think I'll mention it here since I did see your post about feeding pe mysis to your other lion, and mentioned feeding with a baster. While really small lions can be satisfied feeding foods through a baster or even feeding foods like pe mysis; over the long run they will literally slowly starve to death. Lions will not live very long eating small foods in the water column, and when you do feed with a baster, they literally need to come right up to the baster and continually just eat from the end. After a feeding, you should see a nice little bulge in the belly, lions eat on gorge/fast schedule and should be given time between meals for digestion. They need chunky pieces of food like salmon, scallops, shrimp, and any seafood they will take; including mussels, squid, etc. Stick feeding is a good practice as well as getting them to come up and take the appropriate sized chunks as you drop them in.