If you put yourself in his paws for a second, here he is in a strange place.
Different smells, different furniture, different layout - and - most of all - NO MUM!! No BRUVVAS!! NO SISTAS!!
You were all ready for this little chap - very excited and full of anticipation.
But all he knows is that what was familiar, and family, is no more. He will come to know and love all of you in time, but just now, he needs comfort, reassurance and attention. He needs to know where he can go and poop and pee, and play. He needs to know whether the two strange other cats he can smell will eat or attack him, or whether they may be up for a spot of snuggling and playtime and showing him the ropes.
If they won't/can't snuggle just yet, then you need to be there for snuggling duties, and to show him you're his new Mummy now. I used to read fairy stories to our kittens, and make up stories with their names in them. I'd turn the lights down low, and make sure there was nothing loud on the T|V or tablet, and then hold them, stroke them and encourage general sleepiness, holding them so they'd know they were safe.
Couldn't do that with the girls of course, due to their feral streaks, but I would sit in the same room as them, lights low, and tent my knees up with a blanket over them. They would then crawl in under the blanket and go to sleep resting against my legs. As they got braver, then they'd venture up on to our laps.
The great thing about having a kitten when you have older cats too, is that they have a role model to take their lead from. As long as you take the introductions nice and steady, they have the potential to become lifelong friends.
You got any piccies of little Remus?