Struggling with first foster
I'm new and fostering with hopes to adopt a special needs Aussie after the loss of my 12.5 year old mini Aussie abit over a month ago. I have a 27 lb mini Aussie, he's 10 but still very active and my heart dog, we lost my partners dog and he quickly fell in love with this new dog so I'm trying very hard to make it work for his sake and the foster is a very sweet dog who's never had a good moment it doesn't seem.
He was so shut down at the shelter they had to pick him up and carry him into the meeting room and once he was there he wouldn't stand up on his own for maybe 6 hours after coming home with us. He's 47 lbs, stands the same height as my mini Aussie but mine is all legs and twiggy at 27lbs. I had concerns about their size difference but that hasn't been any real issue so far, we've had him a week and I know that's early in this whole situation but now that foster is moving around a little on his own and letting us pet him he's pushing my boy away from us to get more pets. I'm only willing to keep him if it doesn't make my boys life harder, he's been through enough losing his brother who he had never been apart from and I only was willing to try to foster this one for my husband and to hopefully give my boy a friend but so far they have sniffed each other but not more than just "hmm you're still here" vibes to each other, a few crinkly nose teeth showings but no snapping or growling at each other. I don't want to give the wrong impression, I like the foster alot but I have to put my boy higher on the priority list and I think that's reasonable. I know this takes time but I'm not sure how to proceed. We aren't able to do the neutral grounds walks because despite being at least 2 this dog has never been exposed to a leash before and has some feet injuries we are working on but he's very sweet with us and let's us touch feet and I even clipped toenails today(the rescue he was at doesn't know how backstory but I suspect he was a stud dog with very minimal human interactions and mostly crated)
Worth noting my boy always deflected dominance to his brother, he's never wanted to be big dog leader guy so he's not really pushing back on foster but I also don't know how to tell a 10 year old dog he needs to push back.
It's been discussed with my partner and he knows that if this isn't resolved foster can't stay but I'm not sure how to kind of train/guide the foster out of shitty pushy natures when he's so damn skittish to begin with. Either way we will probably keep working with him until the rescue finds him a home just because of how shut down he was in the shelter but my gosh I'm not sure how to be good with foster who definitely needs a good home without my boy feeling slighted. Thanks for any guidance<3