My oldest daughter was the same way, and I too worried that she was sleeping too much. But, her pediatrician never seemed concerned by it. I remember one morning when she was probably 9-10 months she hadn't woken up and it was lunch time (she had gone to bed around 8 pm). I kept peeking in on her to make sure she was breathing. When she finally woke up she was in a great mood, and hungry, but otherwise great. She still took an 1-2 hour afternoon nap. Her regular schedule was wake up around 8-9 am, nap in afternoon from 1 or 2 until 5 pm, then bedtime at 8 pm. Unfortunately, my second child was the total opposite. She slept on average 9 -10 hours total a day, including naps. My daughter is 5 1/2 now, and is a very healthy, active and smart little girl. Her sister, 4, is also healthy, active and smart. They just require different amounts of sleep.
So, I think if your baby is otherwise healthy and thriving, he should be fine. Everyone has different sleep needs. Hope that helps.