Hi there,
I'm not sure what time he normally goes to bed, or what time you're thinking of doing for an early bedtime, but the reasoning I can see of an earlier bedtime on one nap is that he is waking from that afternoon nap earlier than he would wake on two naps, so if you keep the same bedtime, that may be too much A time for him. I don't think he'll be getting too much sleep, unless you're putting him to bed way early, in which case he may start waking early in the morning....
But... if one nap means a much earlier bedtime because he can't handle the A time to a normal bedtime, then... you may still want to do a small morning nap and a proper afternoon nap. The consolidation of naps into one nap should coincide with when baby can do longer stretches of A time, I think 4.5/5 is the "typical" window.
Something like this:
7am - wake
11:30 or 12 - sleep for 2 hours
7pm - bed
The 2-to-1 transition, from what I've been reading, can take a long time to implement. We're planning to gradually shorten and shorten the morning nap, from 1 hour, to 30 minutes, to 20 minutes, until we gradually phase it out, then bump up the afternoon nap (which is normally around 1:30-2pm) to just after lunch.