My daughter is 14 weeks old and doesn't have a solid pattern. Some days her emerging teeth give her more trouble than normal so she'll sleep less because she's sore and it wakes her up. But at that age, they should only be 'really' (of course, every baby is different) having a morning nap, then an afternoon nap, then a big night sleep (broken of course). My daughter has started being really obvious when she wants to sleep so I put her down and off she goes!
I'm confused, you expected him to sleep from 8:30pm until 4am? Has he done it before? My daughter sometimes does 7 hours in a row, but only if she's happy enough too.
I also co-sleep, I put her into bed with us when she wakes up for her first feed during the night. Works a charm! She sleeps better and so do I!!
Good luck :) Hopefully it was just a 'today' thing