Just a few thoughts...
Pack orders change. Boy and Fynn are rescues. Their lives have been turned upside down on lots of occasions. They have to figure the new situations out each time they enter one. That takes time. Not weeks, but months, sometimes years.
Every time I bring a dog into my house, things change. Sometimes it's not a new dog but internal pack order changes and seems to turn upside down again. Then straighten out. But will be different from before.
Boy maybe just getting used to his new surroundings and trying to gain position. Fynn sounds like second in command to me. Boy sounds like he's in charge. However nicely, but defiantly in charge.
You have "loved" Boy so much you might have been sending the wrong messages. Letting love get in the way of discipline. I know all about that...It happens here all the time.
Wanting Boy and Fynn to be perfect buds may take years. I know my mentors would be asking me....What I want to change and why.
When Boy looks at you is he deciding who's in charge? Then you get nervous and could be sending the wrong message? Thinking about the vet trip, you are/were worried, Boy picks up on that and worries with you. I'm not considering the actual medical issues that might, or might not be going on, just the head stuff.
Take a step back from the situation, try not to worry about the psychological stuff, rule out or rule in the medical stuff. Then start a new day.
I have two adult daughters. There were times when they got along perfectly, and then there were years...and I mean YEARS...where they hated each others guts. Point being...things are always changing in a family. Be it Human or Dog!
I'm thinking about you and how you?re feeling. Sending positive thoughts your way.
Why don't you take a break from the dogs today or tomorrow, go shopping, or swimming, do something without the dogs. I know they are your life, but you all could use a break. You'll be happier to see them, less stressed, and so will they. Remember absence makes the heart grow fonder. Or at least clears the mind from the present.
Hope you have a relaxing day.