Its tricky! Here's some info:
http://babywhispererforums.com/index.php?topic=85130.04-6 months Two naps, 2-3 hours each OR;
Gaining mobility 3 naps; 2 2hr naps & 1 45min catnap
10-12 hours at night
6-8 months Two naps 1-2 hours each;
More mobility; able to 10-12 hours at night
sit and crawl
By 8 months the CN is usually well and truly gone. TBH if I had a short second nap, I'd just try a slightly earlier bedtime. Do you think is your LO ready for a touch more A time? If you find that that second nap continues to be short it could be he's just not tired enough to sleep any longer.
More info:
http://babywhispererforums.com/index.php?topic=64158.msg476652#msg4766526.5 - 7 months 2.75-3 hours. Some are getting more.
8 - 10 months 3 - 4 hours. Some are getting more.
Your LO is on the lower end and could maybe managed 10/15 minutes more. See how the nap goes for a few days though and see if it continues to be cut short.
With the bit more A time a short nap won't matter so much as it won't be such a stretch to bedtime and you'll beable to keep it consistent.
HTH