Is he happy & normal through the day? If so then It sounds to me like he's just got older & needs slightly less sleep in 24. At that age my DS2 would only sleep 10 hours at night & 1.5hours in the day, where as my DS1 was still on 2 naps... every child is different, it's more about how they function when they are awake than the exact hours.
You could try making the day more physically active for him to tire him out more or cut the nap shorter/bed a bit later, but apart from what you are doing in not getting him up, there isn't much else that worked for us - until DS2 hit High School this month & now he sleeps in past 7... took over 12 years to get it & it's when we don't want it LOL