Ok, I would keep with the 930 morning nap FOR NOW. Wake him at 1030, though.
The point of shortening the morning nap is sort of two-fold.
First, if LOs go to sleep too soon after they wake up AND sleep for as long as they want, usually they end up with EWs. Their body sees the morning nap as an extention of the night sleep and the wake up time as sort of an "interruption" and in order to get some good quality awake time, they start waking up earlier - he knows that he can go back to sleep quite soon and sleep as long as he wants. So when these EWs start happening, there are two solutions (or a combination of both). Which are to push the morning nap later and/or cut the morning nap short. The reason I suggest cutting it short in your case is because he is already having it at 4ish hours of A time which is a LONG awake time for a 10 month old, and so pushing it later will just make him more OT which will build through the day. But, if you start cutting it shorter, say 930 - 1030, then over the course of a few days, he will start realising that he can't just sleep as long as he wants so perhaps he should sleep a bit more in the morning.
But, the key is NOT to cut it hugely at once. His body is used to that big sleep and if you were to cut it to, say 45 minutes, at once, it would make him crash, you'd get more OT and more EWs and NWs and so on. Cut it for 15 minutes and give it a few days. If after, say 4 days, he is taking a good 1 hour morning nap, a good afternon nap (at the same ish time every day) and going to bed at about 630, and he's STILL waking early, then you would cut it by another 15 minutes. Does that make sense?
The second part of the reasons to shorten the morning nap is in preparation for the 2-1 switch. Which, I don't think your DS is starting quite yet, but probably will soon. So, the faster we can help get these EWs sorted the better! With the 2-1 transition there are a couple of ways to go about getting LO down to one nap, but one of the ways is to keep cutting the morning nap, moving the afternoon nap earlier and LO will be tired enough to sleep a longer afternoon nap. By continuously cutting the morning nap shorter and shorter (over time, as LO gives signs s/he is ready!) the afternoon nap becomes earlier and longer until there is no morning nap left!
Any of that make sense or is it all rambling? !!