When he barks I really wouldn't go down and stroke him to 'reassure' him. What you're doing is telling him that every time he barks you'll give him lots of love - which of course he thinks is great. Unfortunately he'll start thinking that as barking at night gets you back for love, every time you go out the door he'll think it's the answer to get you back for lots of love too - and then you'll have a dog that is a real problem. Also by reassuring him you're telling him that there is something to be concerned about.
I would go down stairs and wait for him to stop barking for a second and then go in. Don't give him love but consider that he might like some blankets - it has got colder recently. Don't fuss him, put the blankets round him tell him 'bed time' and go upstairs again. If he starts barking again go down cover him with blankets again say 'bed time' and leave again. Then next time stay outside the door and say 'bed time'. It might take a while but you will get there if you persevere.
I dog sat for a dog who was getting up through the night because the owner thought he'd trained her to bark to let him know when she needed the toilet. What he'd actually trained her to do was to bark every time she fancied going out for a sniff around the garden. I ignored the random barking in the middle of the night and she didn't bother again - dogs can be very fast learners, but don't always learn what you think they're learning!
If you do have him upstairs, be aware that that might not be enough for him either and he'll end up barking to be in bed with you or just climb in during the night anyway. I've experienced that with a dog with no boundaries at home who also sleeps in their owners room. Personally there's no way I'd be starting that.
I rescued a Whippet that had apparently slept in it's previous owners bed, He was happy down stairs with bed, duvet covers and blankets galore. I'd definitely be seeing if that is the issue before anything else.