S - we struggle with the same thing with Ian, 5.5 months. Currently his bedtime is 7pm and he wakes to have a little crib party between 4 and 5. It lasts over an hour. I have experimented with everything - go in, don't go in, feed, etc. I get the best results with a little bottle (3 oz) but sometimes the party goes on. I guess he just needs to exercise his voice (which currently is a horrible screech!) Today, though, he was up at 5:45. I tried the bottle but nope, he was up for the day. I do try to leave him in bed until at least 6:30am.
I read on another post to try moving bedtime up 30 minutes to 6:30pm - I may try that for a while. It is a struggle to keep him up from his last nap until bed time.
I wish I could be more help - at least we can be supportive of each other until the NEXT phase of whatever hits!