Hello!! I was a newbie too a week ago. The shin and complexion of his scale and fins make him look healthy. You need not worry about lethargy if your betta is just trying to adapt to a new environment. I would recommend a properly furnished tank as his home, like natural aquatic plants, smooth pebbles, synthetic plants (avoid sharp points and edges) and many more (not including metal).
Welcome to the forum and best of luck taking care of your fish. =)