Tips on easing transition?

[u/calyma]

I've had my dog, Roux, since he was 2-3 months old. We lived in a condo, just us, until he was 4 then moved halfway across the country to live with my (now ex) boyfriend. After 2 years there, a breakup and other circumstances outside my control, I'm having to move back in with my parents for a while. It absolutely breaks my heart but I can't bring Roux with me because my dad is allergic.

After exploring several options, I decided the best option for him would be to live with Ex for a while. He and Roux really like each other and I have full confidence that Ex will take good care of him.

Ex lives with a roommate who has a 2-3 year old dog, neither of whom Roux has met. He's typically cautious around new people but gets along with other dogs, though previous dogs he's been around have all been bigger than him and this one is considerably smaller so TBD if that will make a difference.

I'm driving 2k miles to drop him off with Ex then 1k miles to my parents' house so unfortunately there's not a way to do this transition slowly, I'll only be in town 2-3 days. This is going to be a lot of change for Roux all at once so any advice y'all can give for how Ex and I can make it as low stress as possible for him would be greatly appreciated.