Warming up to a stray dog

Long story short my road is known for dumping stray dogs so I come home with them all the time lol. I recently saved what looked to be a 2 year old lab mix and I just knew I had to keep her, but I already have a pup, so I called my boyfriend and finally convinced him to keep her. The only problem is he is worried that she won’t warm up to him. I kept her for a good few days before he took her home and she warmed up to me well. She was wagging her tail, barking at birds or lizards, and licking my hands. At his house she just lays there and the only time she perks up is when she goes outside. He’s never had a dog before and I can tell she’s going to be a work in progress because I’m sure her previous owner treated her very bad, but does anyone have any tips on how to get a dog to warm up to you?