Just a couple of thoughts...Do not in any way force introductions. How is your little boy usually with strangers? If Loki wants to hide in the other room for a while, allow him to do so until he feels comfortable. First intros should be at eye level with Loki. Even better would be Loki being higher than your BF. Height is status to cats, and he is likely to be intimidated by the big man who has suddenly arrived in his territory. Tell your BF not to stare at him, this is seen as confrontational, but perhaps to look at him, blink a few times, then look away.
If Loki should go to hide, after a while, you and your BF may want to sit in the floor in the room with him and speak softly. Just allow him to come out in his own good time. Perhaps Loki isn't one to go hide, in this case, I still feel it's very important, especially if your baby isn't used to being around men (for instance, Sierra is afraid of men), to have him speak in a soft a voice, not a whisper, just gently, from the time he arrives to your home. If Loki will go for it, let your BF serve him his meals, give him treats, etc. That should help win your baby's approval for sure!
Super important is to not allow Loki to feel left out in the least! Make sure he gets every bit of the attention he's used to having, if not more. If the two of you have special routines, stick to them as closely as possible.
Hope all goes well, and I look forward to hearing about it! Enjoy your special time!