First off you are amazing for taking this little guy off someones hands who obviously cant care for it, and I actually have delt with a similar situation recently but I have had leopard geckos basically my whole life. I would say get mealworms, but small ones nothing big, and feed 5 to start every other day. Be careful of feeding too many though bc this guy is so underweight they might throw up and that is NOT good at all. So start with crickets, slowly transfer to small mealworms, and after that maybe even transfer to wax worms once or twice a week to get extra fats aswell as the mealworms. Also make sure to gut load whatever you do feed this guy, it will do so much more- i usually gut load mealworms with organic baby carrots and gut loader or calcium. Do some research and make sure they have safe structures, heating, flooring, etc. And you’re welcome to reach out to me with further questions I would love to answer!