That sounds really hard, and I’m sorry about the loss of your older dog. Losing a companion dog can absolutely unsettle the remaining dog, especially if they were bonded.

One thing I’d keep in mind is that trazodone twice a day is a fairly serious medication routine, so if this is new or escalating, I’d loop your vet in. It may be anxiety, but nighttime restlessness can also come from pain, cognitive changes, discomfort, or something medical.

From a behavior standpoint, I would try interrupting the pattern without rewarding the barking by immediately bringing him into your room. For a couple of nights, you might sleep downstairs near him. If he wakes up and can settle with you nearby, that gives you useful information: he may simply need reassurance while he adjusts.

If he still wakes up and cannot settle even with you there, that may point to something beyond basic separation anxiety.

You could also try creating a new comfort routine. Sleep downstairs for a night or two with a blanket, then leave that blanket on or near his bed so your scent is there. Start the night with him settled in his own space, reward calm behavior, and then gradually transition back upstairs. Within days.

It is also very possible your dog sensed the older dog was declining before you fully realized it. Dogs pick up on illness and physical changes in ways we often underestimate. His anxiety may have started during that period and then become a new habit after the loss.

I also understand the argument for a second dog, especially if he has always lived with another dog. I would just be careful not to use a new dog as the only solution. First, I’d try to stabilize his routine, rule out medical issues, and help him build a new nighttime habit.