I like the bucket style helmets much better for bike seats too. I find that in general the just stay in position much better for little ones. We have a firm pillow behind our guys back. It move him forward just enough to give his helmet a tiny bit of a shelf to sit on. Then if he falls asleep, the pillow "shelf" keeps his head leaned back just a tiny bit so his head isn't bobbing around.
I know that some bike seats recline, but the aren't widely available here, mainly in the UK.