I would persevere into getting him onto 1 nap - you may need to bring lunch forward and get his afternoon nap at 12ish - If your are satrting to get him to toddler groups, they are usually mid morning so that the kids can go, have their lunch and then have their nap, so you will find that this fits in better with life in general. either that, or you need to start the evening routine a bit earlier and string it out a bit so that bed time isn't brough forward too much.
All babie/toddlers are different, but I think that my kids were sleeping 11-12 hours at night with a 1-2 hour nap.