I am so sorry for your loss.
Your left behind glider may have been aware of his brother's passing, if you found the body in the morning it is highly likely. He still may be wondering where he is or what happened to him. When I lost my Lulu, I felt her cage mates looked at me with suspicion and distrust. It was like they held me responsible for her disappearance. But that could have been in my head, I didn't find her the first thing in the morning, it was a little later in the morning and like you, can't be sure they knew of the passing or not.
I do not know for sure, luckily I had a trio, so the passing of one at this time didn't cause me that same concern you have. I believe a little time for him to grieve should be given, but it can take a while to find a new mate, and then you may need/or want to do quarantine, depending on where you find a mate for him, or to make sure he's healthy first. Quarantine is 30 days, and this starts after first fecal at vets, so this can actually take quite a bit of time. Then after all is well, there is the intro preparation, and that can take usually around two weeks, possibly longer. So, it might not hurt to start your search, but I wouldn't commit to any until you get your nec results back or have your little guy checked out or even both.
In the meantime, give him as much attention and love as you can, he's scared and probably feels very alone suddenly. And please keep us posted on how both he and you are doing and any news from the nec, as well as your search for a new cage mate for him.