Boy, that is a tough question. I know that you are very well educated about how rats should always be kept with at least one buddy for companionship, but once in awhile, an observant rat person will notice that it just seems more stressful with certain rats together and they ultimately decide to keep a particular rat on its own when it seems the rat does better and is less stressed out than when they are with other rats. It does happen on occasion.
A tough call to make! If all attempts at getting a good relationship started do not work out and your boy just seems to be more comfy and happy without other rats then that is a call you need to make, as you know your own rats better than anyone else. It does seem very strange to keep a rat solo, since we have all learned (well hopefully all of us!) that rats need to have a buddy to thrive and not become lonely. But, like I said...if it only ends up stressing him out, then maybe he will do better on his own, as long as he has lots and lots of attention and interaction with you daily.
I feel so bad for Sebastian, looking all over for Sully. I know how heartbreaking that is.
Our previous boys, Klausse (a Russian blue dumbo) and his brother Wolfgang (a brown agouti dumbo) were so close, and when Klausse developed myco and we ended up having to put him to sleep, I remember that my husband and I gave his brother Wolfgang tons and tons of extra special attention every day after his brother was gone, but we did not have enough time to decide on getting him a new companion, because Wolfgang ended up with increasingly worse myco and he was gone only about a month after his brother. Our veterinarian said he (of course) had the exact same thing as his brother...it just hid out a bit longer, then reared its ugly head very quickly.
So, oh my gosh, it is soooo hard. Hard to lose your babies and hard to see the brother that is left without his buddy and wanting to give him a million kisses a day. You of course know exactly what I mean.