Hi Katie,
The transition to one nap was hard here as well! Mostly because we were dealing with early wakings as well.
First off, I would try maybe putting him down earlier, say even 11:30 or so. It could be that he is overtired. My ds is almost 19 months, and has his nap anywhere between 11:30 and 12:30. The other thing I did, especially on those early waking mornings, was go run some errands in the morning and let him have a 10-15 minute catnap in the car. I found that this really helped to take the edge off, and therefore he wasn't so overtired by the time his nap rolled around.
It took us a few weeks before he eventually settled into his one nap, and now he naps for 2-3 hours everyday, and still goes to bed at 7.
Good luck, and I hope this helps a bit!!
Carrie