I second the idea of going on walks with him. One of the reasons he resists going outside with you may actually be because he was kept outside all day. He has no reason to believe that you won't leave him there. He could be afraid that if he goes out, he'll stay out and he'll lose his chance to spend time with you inside again.
Taking him for walks is a great bonding experience. It will help him learn to trust you and understand that he comes back inside after the walk. This could then help him gain a little more confidence and freedom in going out by himself.
We got our dog 10 months ago. The shelter told me to walk him even before I took him inside to the house. What a great idea! I had the rest of the family greet us in the front yard, then took him around the block. It helped to settle him down and build a bond from the beginning. Still, it was only a few weeks ago that he started going outside without me, and that is only for a few moments. I don't think he's ever done his "business" without me outside.
Be patient and kind and he should come around. So much to adjust to! New people, new being inside, new doggie sister, new life. For a dog that has never been trained it can be quite confusing. Walk him a lot and then just take him outside frequently, use a go potty command and by all means, stay/play with him. Since he is small, you might want to carry him out for a while until he wants it to be his idea.
Good luck! And kudos for giving a dog a second chance at the good life.