I am not generally a fan of cry it out, but sometimes, it's the only way. But, in answer to your question about nap time, I actually think that it is more reasonable to let him cry it out during the day than at night. Although, 4 times a day is a lot of crying to listen to. But then, for me, nighttime crying it out was not an option because I personally can't take it.
I tried the cry it out during nap time for my son at that age, and it just never worked. I figured that he wasn't ready for it, and didn't really become successful at just letting him cry until he was about 15 months old. Before then, he would become more and more agitated and then never go to sleep(and I really mean literally never, not even for the night, rather than literally "crying it out" and then calming to sleep. Just a thought.
After reading other comments, I just think it is so funny that people just assume they know your child. I have 2 boys. They are radically different sleepers. My oldest didn't sleep through the night, ever, until he was 15 months old, and not consistently until he was 18 months old. We tried every conceivable method. He just had to work into things on his own, and he wasn't too terribly fast. The second one cries it out already, and has been since he was 3 weeks old (he's just shy of 3 months now)... and has been sleeping through the night since then, as well. This is not because WE are so much better with him. This is because HE is a different child than the first. He is very laid back and rarely cries, ever. The first one cried all the time and is incredibly sensitive and high-strung.
You do what works for your child and yourself. I am a huge fan of doing whatever is best for your kid. If rock or nurse to sleep is best, do it. If crying is best, do that. I think we need to be flexible with our kids, within reason, especially when they aren't even old enough to speak or understand consequences.