I hear three issues here: won't Come, nervous, and not potty trained.
Come you can work on. It's actually very common for little dogs to fear your approach, and almost all dogs love a good game of Chase more than they like the Come command. There is a lot of information on this forum for training a strong recall. There are also things you can go to help him learn human body language so he can understand that walking straight up to him is not a threat.
Nervous: as recommended, walk, walk walk. Chase him around the house if he likes it (though that also stimulates peeing and pooping, so maybe not).
Potty training: you've been given a lot of advice already. Here is my 2 cents. When I started potty training Thor, I did not fully commit to it because like you, it just sounded like overkill. It took me about two months to accept how much work it would be. I know it sounds crazy, but I took two weeks and really did NOTHING but watch him, with BOTH eyes. I had tried just keeping ONE eye on him, and he would potty so fast I would miss it. It was way harder than I expected, and I definitely thought at one point he would never get it, but at the end of the journey... you have a potty trained dog. It's worth it.
