Is your son a preemie? Preemies tend to need more sleep longer than a full term babe.
If not, it could simply be that your DS requires a little more sleep than most babies. I wouldn't worry about it. It took a while for us to push our son's activity time too. I actually started with 5 minutes increment. I would keep him up for 5 minutes more, ie. 8:05, before bedtime for three days before uping the anty another 5. I did have days when we had to go back to the old bedtime, (8:00). When I reached 15 minutes (8:15) more of A time, I started pushing the anty another 15 minutes. It was harder, but with lots of funny faces, looking at the chimes, miror or outside. We managed it. It's hard to change the internal clock and he will be cranky/tired a bit, but go at your and your son's pace.
Most important do what you feel is right for him. Again, I wouldn't worry. Just try pushing the A time more slowly for him.