I have had a lot of the feelings you describe... initially, watching my husband with another woman was extremely surreal, and not even necessarily in a bad way. When our GF and him began alone time, and that involved sex, I remember being like... wait, huh? What? My husband just had sex with someone who isn't me. How did that happen?
Also, the first time I was alone with her... it was bizarre for him to not be there. When you spend 10 years kissing one person, it's just strange to kiss someone else, even when the person you HAVE been kissing is there, and it's all permitted. I sorta felt like I was cheating, even though I *knew* I wasn't. He expressed having had similar feelings during their initial encounters alone.
Now, it's all totally normal for him. He's been Mr. Compersion since Day One, lucky duck. I am a lot better than I had been, but I still get a twinge every once in a while.
I wish you and your BF all the best going forward.
Keep us posted.