Hi Suzanne,
I would have his pm nap be about 3 hours (or whatever his ideal afternoon awake time should be) from the time he wakes up from nap #1.
One tip that you might want to consider, now that Taylor is older and needing less day sleep, we have been limiting her morning nap to no more than 1.5 hours (and don't let her sleep past 11:15). This way nap #2 always falls between 2:15-2:45ish (with 2:30 being the average). By limiting her am nap, it pretty much always ensures a good pm nap of at least 1hour 15 minutes-1 hour 45 minutes so that she can make it through to bedtime.
HTH!