I have some thoughts, but I'd first like to know if he is tired or cranky before nap time. I'll go ahead and mention to possibilities:
1. My 10 mo old goes down for first nap 2.25-2.5 hrs from when I get her up. She takes a few more minutes to fall asleep, but then sleeps 1.25-1.5 hrs. If your little one is overtired by naptime, this may be why he's not sleeping longer. And, if he's not rested enough from the first nap, he'll have a hard time sleeping during the second.
2. However, as the other poster mentioned, he is getting alot of night time sleep, but it may be because he is tired from the day. I think it might help, as stated, to slowly decrease his night sleep (say in 15 min increments). He might be sleeping less for the am nap because he is not tired enough. I would suggest to keep the same am nap time, but either get him up earlier in the morning or put him down later for night time. That being said, it will be hard for a little while b/c I'm sure he's exhausted at night and ready for bed. But, once you can sort out the night time sleep length, his am nap should improve, and thus his pm nap will also improve. I believe it's a related cycle.