J.R.
I think it's fine to give them different bedtimes because they are so far apart in age. You could always save 1/2 hour of homework and use that as an excuse as to why he has to stay up.
Also, my kids share bedrooms and I don't allow talking. Their room is dark and they have a cd player playing. If there is talking going on they know they will lose their music.
I also tell them that every time I have to come in there I'm taking 10 minutes off of the next night's bedtime.
Hope this helps,
J.
Mom to 5, ages 7, 6, 5, 4, & 2