Short naps that are less than 45 minutes are indicative of OT, greater than 45 minutes means UT (needing more A time). He seems to be getting the appropriate amount of A time for his age (usually 1hr and 20 -30 minutes), but maybe the first A time needs to be less... It's seen as an extension of night sleep and it usually is less than the A times throughout the day. Maybe he's OT after that first nap and by the time the second one rolls around he can't settle.
Also remember, babies work on their night sleep from 0-3 months and day sleep from 3-6 months. As I mentioned, my DD 2 did this for awhile when she was fighting the OT monster, and it was in that 3-6 month age range, but now she's a champion napper! It's fabulous that he sleeps through the night, and I'm sure he'll figure the day sleep out soon. Just pay attention to his cues and put him down right away at the first sign of tiredness, even if the clock doesn't match up. It's more of a routine (ie things in the same order all the time for consistency) as opposed to a schedule.
HTH
