What time does he go to bed at night? My daughter is also 6 1/2 months and she is usually ready for a nap 2 1/2 to 3 hours after being awake and active. This is her sleep schedule:
wake up -- 7:00 am
1st nap-- 8:30/9:00 -10:00/10:30am
2nd nap-- 1:00-2:30 pm
3rd nap-- 4:00-4:30 or 5pm
bedtime-- 7:00 pm
Of course, these times are just estimates and can vary by 30 minutes or so. The important thing is to pay attention to your baby's cues (rubbing eyes, yawning, fussing) and start winding him down and getting him ready for his naps/bedtime when he shows you he is tired.
According to the book Good Night, Sleep Tight: The gentle guide to helping your child fall asleep and stay asleep, babies that are 6 1/2 months old should be asleep for their first nap around 1 1/2 to 2 hours after waking in the morning. They should be asleep for their second nap no later than 3 hours after they wake from their second nap. Again, pay attention to his cues and if he seems tired earlier (and especially if he didn't nap at least 1 1/2 hours the first time) you can put him down for his second nap earlier. The same thing with the third nap.
The third nap can be optional depending on how well he slept the first 2 naps. If for some reason he didn't sleep 1 1/2 hours each nap then he will probably definitely need the third nap. If he already napped for at least three hours during the day and doesn't appear tired, you can skip the third nap, but maybe wind him down and put him to bed a little earlier than usual.
good luck!