I have a 6 month old son with a similar situation - I guess the difference is that I don't really think it's a problem. He naps about 2-3 hours total per day in 40 minute chunks (usually 3 40 minute naps). And he generally sleeps 11-12 hours at night with 1 wakeup. On occasion he will string two of the 40-min cycles together and sleep for up to 1.5 hours, but that is rare.
I guess the question is what's the big deal if he doesn't nap that much? It sounds like he's getting good sleep at night and presumably you are too. Isn't that what's important? If he's cranky but won't go down just remember to keep him busy beforehand, nurse him, and then put him down (with a clean diaper, obviously). Does he have a "lovey"? That will help at this age too.