Sounds like he has come to expect that to happen...like bedtime happens so they know it is the long sleep, he has become acoustomed to you taking him into bed... the only way to stop it is to not do it... it may mean you spend 2 hours trying to get him back to sleep for 3-4 days, but with 5/6am ish happenings, the longer you leave it the harder it is to break the cycle... it is when they tend to go into the last deep cycle of sleep, but before they do they have a very light sleep cycle... often talk/whimper even if asleep & if they get disturbed a couple of times, then it becomes a bit of a problem... like you have now.
Unfortunately there really isn't an easy way I know of to break it except to do the hard yards :cry: