Hmmm, a few thoughts. I don't think he's getting too much overall sleep although you could maybe expect to see something less than 12 hours at night--I think most lo's around 18 months get about 11 (except mine who wouldn't do more than 10!) so that may be part of it. He still needs to get second year molars and while it may be a bit early for them that may be it. My older dd got all her teeth early and had a full set by 19 months so he might be teething. Alternatively, ear infections may not show any symptoms except when a child is lying down (due to changes in the ear pressure) so you may want to get that checked out.
He's been on one nap for about 5 months now and I note that he used to go down for it about 4 hours after wakeup and he's up for about 6 hours after nap. I think many lo's when they first go to one nap need to go down pretty early because they just can't make it that long before having a nap but once they adjust to one nap they do better having the nap about 5.5-6 hours after wakeup--if he's waking at 6:30 or 7 he's not quite getting that, so he may just need a nap slightly later. There's a couple of things you could try--since he's not getting as much sleep at night, you probably won't want to move the nap later at first so move his bedtime slightly earlier--about 30 minutes. With my dd night waking and early rising was usually a sign of overtiredness/putting her down too late at night. So if you do that you may find the night wakings go but he may still wake around 6:30-7 and want to nap at 12. If the earlier bedtime works fine for your schedule, then you may want to go with it but if not, just move the bedtime earlier temporarily, then you can slowly shift both the bedtime and nap time forward 30 minutes or so maintaining that morning wake time closer to 5.5 hours before nap with bedtime 5 to 5.5 hours after nap. HTH
