How do you know if your baby is ready to go from 2 naps to 1?

My guy tends to have 2 short naps. First, 30-40 mins around 9am, and the second, about the same at 1:00. It can be right to the minute sometimes. I have tried to skip the morning, but he didn't lengthen the afternoon. It was just 40 mins, and he was a crab. He sleeps pretty good at night. Usually from 6-6. I wish I could get him to go 7-7. What worked for you?