Python, 5959 58 bytes
I'm sure there's no real way to beat the Retina version, since this is basically just that within Python. But I think this is my first submission ;)
import re,sys;print(re.match('^[A'[A-Z][a-z]+$',sys.argv[1])) It's a very odd truthy value:
(test2)wayne@arglefraster ~/programming/inactive/golf/67554 ⚘ python golf.py AdNan $? 148 %# 3 10:06:36 None (test2)wayne@arglefraster ~/programming/inactive/golf/67554 ⚘ python golf.py Adnan %# 3 10:06:40 <_sre.SRE_Match object at 0x7feefea7f440> (test2)wayne@arglefraster ~/programming/inactive/golf/67554 ⚘ python golf.py "Adnan Banana" %# 3 10:06:47 None (And it does require "" around strings with spaces in it, if passed via the shell)