In my own case, every time ds wakes up crying means he's not rest enough. It never fails for us but each lo is different. Given your ds's age his schedule seems pretty fine to me and if he's not waking at night, i doubt he's overtired.
However it's been a big change from about 4 hrs A time on his past schedule to 6 hours in the current one. Even tough you mention that bringing the nap forward is not heloping him sleep longer it might contribute to help him go down easier. I'm thinking he might be a bit overtired when he goes down for his nap. Maybe putting him down at 12:30/45 might make a change.
On some ocassions, teething is a big disturbance. He's in the right age to teeth his 2 year molars. Might be worth a shot to offer some tylenol before each nap and bedtime and see if that helps.
I don't think moving him to a toddler bed will help at all. In fact, it is likely that the change will contribute to more wakings and more stress on his side. Unless he's umconfortable in his bed (which i doubt cause he sleeps a full 12 hour stretch on it) or you need the crib for a coming sibling, I'd suggest you to wait for the toddler bed transition until you have this napping issue figured out.
Along with this and taking advantage on his talking skills, talk to him about this. Try and make him tell you what's wrong, why is he crying. LEt him know that even if he wakes it's ok and nothing's going to happen to him. Buy a special teddy/stuffed animal that "takes care" of him while he's alone in his room. Verbalizing things at this age is extremely important, naming his emotions. Just ask and double ask until you get an answer (you might be suprised to find out that he might be cold, scared, confused... whatever).I'm thinking more of a developmental/conductual thing than a sleep issue cause on average, he's sleeping a good amount given his age.
Hope some of this give you a hint and if i was no help LOL let me know and we'll think of something else. Good luck and hugs!
