Since he already takes a short am nap, I think actually you could make small adjustments that the nursery should be able to do as well - especially since he's only in nursery in the am - because eventually, that's the nap that will go. Can you ask them to simply put him down 15 min later than they currently do? Then when you pick him up, you'll want to try to extend him 15 min as well. He really is soooo close to what my dd does on 1 nap days, so I don't think it will be a huge adjustment for him.
Even if nursery were able to move his nap by 15 a week, then pretty quickly, I think, he could transition to 1 nap. Now, there may still be days he needs that morning nap, so you could tell nursery something like - if he's really sleepy around 9ish, go ahead and let him take a nap (but no more than 1 hr). (once you've moved his 8:30 nap to 9).
What you can do during the transition is put him to be earlier - try 30 min, you most likely will not get an earlier wake up. Keep putting him to bed early while working on the nap thing. With Abby, if shes way tired an takes a morning nap, I try for an afternoon nap, and if that's a bust, she still has an early bedtime.
I know that's kind of rambling - so, you can do early bedtime, ask nursery to move nap 15 min later and keep it to no more than 1 hr. Then you can move afternoon nap 15 min.
The transition is different for every bub - we've been working on it for almost 4 mos now, and she still has 1-2 days a week where she needs 2 naps (like today), and sometimes still has early bedtime. I know that she needs at least 2.5 hrs of day sleep and she cannot handle more than 4 hrs of A time before bed. So that means if she's up before 3:30 or has less than 2.5 hrs of day sleep, she goes to bed early. But it took me probably the first month of the transition to figure that out! But once I did, that made it much easier!
HTH
michelle