Need advice - Do my dogs like each other?

I have an older rescue mutt, about 6-7 years old, named Finn that I have had for 5 years. He's well-socialized but aside from excitedly greeting new dogs, never seemed super interested in them. When I take him to dog parks he sometimes will play chase but mostly spends the time sniffing and marking the perimeter (he's always done this, even after being neutered). He's not even big on playing with toys (except his teddy bear). My parents have 4 dogs that he sees on a regular basis and prefers lounging in the sun. Sounds like he's perfectly content being left alone but he's always sad when he leaves.

Recently my husband and I signed up to be fosters through a local rescue and took in a dog similar to Finn's age. He occasionally will try to play with her but mostly they just ignore each other. Never fought or had any negative interaction with each other, just tolerate the others' existence. The foster dog started lying in his bed at night with him and he doesn't push her off but he doesn't snuggle with her either. Is he just being polite? I'm seriously considering adopting our foster myself but I just can't tell if my dog likes her.