He's just accustomed to being outside, it sounds like. If you keep feeding him and being sweet to him, he'll get used to being inside.

When kitties of any size first come into strange houses, they're real nervous and they just have to pace and meow and explore everything till they smell all the unfamiliar smells and scope out the territory. They stay sort of low to the ground and sniff everything and creep around. It sometimes takes them a few days to calm down. He'll be especially nervous if he's been accustomed to being outside 100% of the time.

Keep feeding him and just give him the freedom to explore so he can know where he is and know he's safe. He'll calm down. Take him to the vet as soon as you can to make sure he's disease-free and has the vaccinations he needs.