I second Trish on the timing of sleeps... My DS would fall asleep at 4, and I needed to wake him around 6. He would be fussy then, for a bit, but better for night time sleeping, and getting a good routine for when they are abit older...

Hang in there... and remember sleeping through is a stretch of 5 hrs, and it sounds like your DS is going really well with this, you just need to help him shift that stretch of sleep to a bit later in the day. xo