I know it is hard, but be patient and slowly adjust each day by a few minutes. As counterintuitive as this sounds, you may want to actually put him to bed earlier. Don't make major adjustments; just slowly change the times by like 10-15 minutes.
One thing that really helped us was adjusting the 'nap'. Our son is on one nap a day and at this and the last time change (21 months this time, 13 months the last time...two naps then) we just let him have a longer nap(s). For example, yesterday he went down for him nap a little earlier (because it *felt* like 12:30 to him, when it was really only 11:30) but let him sleep til the usual 3:30.
You might also want to try letting him fuss a little bit at 6:15. We did this when my son would wake up at 5:45 - too early, plus we knew he wasn't getting enough sleep. Finally, try consulting "Healthy Sleep Habits, Happy Child" by Dr. Marc Weissbluth. He is a Chicago area pediatrician who specializes in infant and child sleep issues with over 30 years of research and studies involving infant/child subjects.