Kmg - how are you doing? I missed your posting a couple of weeks again about your youngest not sleeping. Did you manage to sort it out? How is the dummy thing going too?
If you've cracked the dummy then how about trying him in a bed? Maybe it would help him sleep if he thought he was going in a bed just like his big brother? I don't know what the age difference is, but could you treat them the same when going to bed now? Put them both to bed at the same time and read a story to them both. If his older brother sets a good example about going to bed then he might follow suit. This may take a few days to get used to a change in routine, but would it help at all?
In my experience any changes involving sleep training have taken around 5 days.
Let me know how you are doing anyway.