You have my sympathy, as mother of another non sleeper!
I do think the obvious thing that stands out from your post is that you used putting him in his room as a punishment (time out). At that age that might well create a negative association. I know you are stressed and have little 1:1 time, but that maybe makes this more important - could you (or your DH) spend some 1:1 time in his room playing with him, to create nice, non bedtime associations to "overwrite" the scary/ punishment association (even if it hasn't been created by using it for time out, it could make the room feel more like somewhere he wants to be).
Have you tried letting him keep a brighter light on? A bedside light or a lego torch (thy dynamo ones are excellent, all my kids have them, they like them because they are under their control, and as there are no batteries they can keep them on all night if they want. The lantern ones are brighter and not dynamo but also last ages even if left on all night). He is also old enough to be allowed an MP3 player with stories and music to listen to when he wakes up if he wants.... Or to go to sleep listening to a CD player and put it back on if he wakes (if your housing layout allows for that without waking everyone). Also (maybe you have already done this) consult him and make a big deal of him choosing things for a room "makeover" (needen't be proper full on re-decoration, but maybe lampshade, bin, blind or curtains, duvet, rug...)
I assume his brother's aspergers is maybe a reason you don't want to put them in together, as perhaps it would upset his brother? If they get on well and you don't want him to come into bed with you and his brother wouldn't mind I'd try putting them in together, so he's not alone when he wakes.
Mind you I have tried all those things except the MP3 (my non sleeper is too young) with no sucess... Still they are just ideas that come to mind, all of which I have used with my older son. My boys share as the older one wanted to and I really hoped it would help the younger, but it doesn't seem to matter :(
Failing that though if you can sleep with him in your bed I'd go with that for now, given it is a stressful time etc. My youngest insists on sleeping on my head if when I end up sleeping with him at some point around 3am every night when I am past the point where I can drag myself back to my own bed so I know it isn't always the solution for everyone, but sometimes, if it does work, you just have to go with what allows everyone to get some sleep!
Good luck!