Sounds like he needs extra A and I don't think he's too young. Some babies start as early as 5 months. Actually DD3 (who is also overdue by 2 weeks and seems to be a little ahead of the av A's) started the transition sometime around 4.5months (the odd missed CN here and there) and by 5-5.5months we were down to 2 naps
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So as you're adding the A's across the day, if you see that it is too late for the CN, you could do an earlier bedtime (6pm is reasonable and we did 5pm some nights to help overcome overtiredness). Or if he does fall asleep but its fairly late or it will push bedtime too late, you could try waking him about 20-30mins into the nap (assuming his catnap is around 45mins at the moment) to preserve bedtime.
See how you go... with us, cutting the catnap any less than 30mins did not help because she was just too cranky and I cut it out altogether and went for earlier bedtime most of the time.
Have a look at this link and let me know if you have any questions.
All about the 3-2 transition- 5/6 monthsHTH