Letting baby cry it out

The first time it happened on accident. I was on the porch with my husband talking and we heard him start crying. We let him cry for a few minutes and was going to go back in but he stopped. When we came back inside he had put himself to sleep. So now I knew that he was able to self soothe I decided to start putting him down for bed a little awake, he did great with that. Naps on the other hand not so great. He’ll cry for 5-10 minutes and then stop for a few then cry again then stop and finally fall asleep within about 30 minutes but the nap only last maybe 20 minutes. Usually his naps and 1 hour long. I have white noise and a dark room so I’m just confused why he don’t stay asleep longer. Any suggestions?