It has been a while since I was making the 2-1 but with DS I had originally had the am nap the shortest. But once I was down to a 20 min am CN, it felt like I could never kick the nap. So I ended up switching them around and it was so much better. What time is your DS getting up in the morning and going to bed?
DS had a schedule something like
7 - WU
S - 12-2:30/3
BT - 6:00-6:30
If you are up around 7 and he's making it to 11, that's pretty good already. You could work to push that am nap back by 15 minutes at a time or so to get more towards noon, which will help with a decent BT once the CN is kicked. Also, you will probably have to cut shorter and shorter after time, though probably not both at the same time. But that is about what we did. Though one day he just stopped the CN all together and we didn't look back. HTH