Little background! My dog used to be fairly socialized, but when he was 2, we moved up to Maine with my family. I decided to move down to Georgia on my own, and for 2 years I spent working and finding my own place to keep my dog. He's now almost 5, and while he was in Maine, he was kept only socialized with my parents two dachshunds.

From this, I've found him now fairly picky over the dogs he likes. He's terrified of almost anything. Plastic bags, loud noises, and specially big dogs. He stands completely still, then snaps at them, as in he's scared.

When we go on walks through my neighborhood, if there's a dog about his size or smaller, he'll run up, remain super still, then he'll play bow, or chase them around and run with them. I don't think he's ever snapped at a smaller dog, except one that was fairly old and seemed uneasy and scared of my dog, already. But I don't think he snapped, he just didn't seem comfortable.

I take him to the dog park sometimes, and in the small dog pen, he does fine! He runs around, and plays, and has a good time. But when a "bigger" dog comes in, he doesn't like it. The last time we went to the park, a BABY pitbull came in. He was about the same size as my dog (25 lbs, he's a JRT/Whippet mix), maybe a tiny bit bigger. At first he was OK, but then he kind of defensively snapped at the Pit. Luckily, he's a baby and didn't really care or know any better, so they moved along. I kept an eye on him, and when he did it again, I removed him from the park.

My friend owns a small maltese like dog, it isn't a maltese, but the breed is eluding me at this moment. But she's smaller than my dog, and older. They've met twice. The first time they met, I was not here. My friend took my dog to her house and they met. Side note, my dog HATES cars. When she got home, she first brought him in her house, and she said he snapped at her dog. Then she took them outside, same thing. So she brought him back to my house. Second time they met was at a dog park, totally fine. We then went out to eat together, on a patio, and they were ok, unless there was food, then my dog got defensive. Basically her dog doesn't give a shit.

I think the first meeting was a combination of me not being there, and the car ride, and the unusual place. Her dog is very passive, and when my dog was living with my parents dogs, he was the submissive one. We are wondering the BEST way to get them used to each other, or to meet, and transition into living together.

TL;DR dogs met twice, my dog snapped one time after meeting friends dog without me there and after a car ride (he hates cars), then once with food in the way, fine together at dog park. Best way to move them in together?