An hour nap is generally undertired. I'd say that is a good sign that he's recovering some of the sleep debt and you could increase the A time now. Could be GS, could be the feed to sleep association, could be ill or teeth moving around. If he woke at 12, it would be PD for next nap 3hr from when he woke, so 3pm. You're working on how long he is awake, not set nap times.
You can wake him earlier or just put him down for his nap later. What are your ideal wake/bed times?